An interesting question.
Some thoughts.
1 - Key date coins are key dates for a reason: low mintage, not circulated, not saved by collectors, etc.
Let's look at CSOTUS's 1909-S VDB vs. '32-S Washington.
There is only one 1909-dated
Wheat cent with an S mintmark with a VDB. There are three 1932-dated
Washington quarters.
In 1909, a (smaller population of) collectors wanting an example of the new
Wheat cent wouldn't have insisted on having one of each type: they'd have grabbed a 1909, any 1909, and kept that as their "type coin" -- maybe a few grabbed a 1909-S instead, if they happened to find one. The plain-jane 1909 VDB was hoarded and sold for up to 100x-500x face value at times in 1909 when it was still brand new.
But although we love our '09 SVDB's today, in 1909, the public was offended by poor Vic's initials on the back being so large. They felt it was egotistical and arrogant. A lot of "average" people didn't bother saving them, if they could ever find one.
In 1932, the
Washington quarter was also widely saved by the public who thought that it would be a great rarity. This was especially true for the P mint, and to a lesser extent, San Fran.
However, in 1932, the country was in a major economic turmoil. The Mint didn't need Denver, San Fran, and Philadelphia all cranking out quarters; so Denver just didn't crank them out much at all.
Up until the mid 30s, you could order a 1932-D directly from the Treasury. (!)
Coin collecting really took off in the post-war economy, and people putting together sets of
Washington quarters, and (a lot more) Wheat cents.
Most collectors of the era tried to put together brilliant, blast-white date and MM sets. They quickly encountered a problem: not only could they not find 1932-S (much less D) Quarters or 1909-S VDB Wheats in change, their local dealers didn't have them either; even the big mail order houses often were buyers and not sellers. They also quickly found out they couldn't find 1936-D quarters either, nor 1922, 1931-S or 1914-D Wheat cents, for that matter.
But those holes sure did look ugly...and prices went up a bit, and would keep going up for years. Much more so on the S-VDB -- at least some 1932 S mint quarters were saved Uncirculated by citizens and collectors; but those S-VDB's didn't get saved Uncirculated much at all. 50 years later, you might have had to hunt for a few days to find an Unc 1932-S, but finding an Unc SVDB was a months-long endeavor and your money was in the market vs. a lot of other collectors trying to buy the same Unc SVDB. Wheat cents have always been more popular to collect than
Washington quarters -- if you consider what you could still get for a quarter well into the 50s and 60s this makes sense...most kids in the 60s couldn't afford to start a Washington collection.
If the US has, say, 10+ million coin collectors, how many of them have started an album of Wheat cents at some point in time?
Now how many of those have tried to put together, say, an album of silver
Washington quarters?
I'll bet the first group is substantially larger than the second.
2 - Coins can be made famous.
I recall a few years ago a TV show of some sort (CSI? Vegas?) featured an 1893-S
Morgan dollar as part of the plot line. Now here's millions of random, non-collector people watching a TV show. They may not know much about Morgans, but the thought is in their head: 1893-S = big bucks! "Queen of the Morgans!" "Ultra rare!"
When's the last time you saw an 1894 Morgan in a starring role in anything except an auction?
Apply this to the 1909 S-VDB vs. 1932 S mint quarter.
Find 1,000 random people off the street who indicate they've ever had even the slightest interest in
US coins.
Ask those 1,000 people what the most valuable
Wheat penny is. How many will say: the 1909-S VDB
Wheat cent? A lot? Sure. They probably know it's very rare, and valuable, and famous, even if they're not sure why, exactly.
Ask those same 1,000 people what the most valuable
Washington quarter is. How many will say "1932-D" without having to think about it for quite some time? When's the last time you saw a 1932-D or 1932-S
Washington quarter being made famous in a movie, TV, play, or novel?
3 -- The Internet has forced us to re-analyze the concept of a key date.
30 years ago, you want to buy a 1909-S VDB. Your local dealer might have one, or know where he can get one. Maybe. But your choices are going to be somewhat limited to a handful of coins. Raw, of course. Let's say you're writing a book on collecting Wheat cents. You know it's hard to find a 1909-S VDB because no one around you has many examples. However, for all you know, a guy 50 miles away might have 50 of them in a cabinet. Your "perception" is that the coin is extremely scarce, because it's extremely scarce where YOU are.
Now, you want to buy a 1909-S VDB. Thanks to the Internet, you can now browse hundreds of listings of coins for sale, whether next door or all the way across the country. Even though the NUMBER of 1909-S VDB cents on the market did not suddenly grow a hundredfold, your "perception" is that the coin is not nearly as scarce as you thought it was, because you've now got a much broader view of the market. It's what we call "market information."
But -- and here's the rub -- those coins have ALWAYS BEEN THERE and they're STILL scarce. You know they exist now; you didn't before; but they didn't suddenly pop into reality with the invention of the Web. You now say to yourself, "Man, those aren't nearly as scarce as I thought they were." But that's not true -- they've always been just as scarce, but your "market information" was imperfect/defective.
This is why you should not assume that just because you can find 100's of listings for any given key date up for sale, that the coin is really 100x easier to find than it was 30 years ago. It's not.
In summary -- as long as people collect albums and sets -- there will always be a demand for "key dates", whether the scarcity is real or perceived, and it takes MAJOR changes in the market to unseat that perception. (Ask an old timer about the 1904-O Morgan situation sometime, and weep as you hear tales of people spending huge money on "key date" 1904-O Morgans, only to have the Treasury suddenly dump millions of the darn things on the market, turning a "key date" into a common date.) Certain coins are so "etched" into our memory and collective knowledge, even among non-collectors, that they are part of our culture -- the 1804 Dollar, the 1913
Liberty nickel, the 1893-S Morgan, the 1909-S VDB
Wheat cent, the 1877
Indian Head cent, the 1916-D
Mercury dime, all are widely famous beyond numismatics!)