Why Aren't Shield Nickels More Popular?

A factor to consider in terms of "popularity" is collector sentiment. Buffalo nickels and even Liberty nickels are "popular" because lots of old-timer collectors alive today remember getting them in change as young'uns, an obsolete coin which may even have sparked their collecting desire. The Shield nickels, not so much as they were pretty much already gone from circulation by the 1960s (especially if they tended not to work in vending machines due to their different specifications). Thus, the two more-modern-yet-still-obsolete designs have a broader base of collectors interested in them.

This. I very well remember getting them in change, and that's exactly what started my collections.

Many people collect Shield nickels as type coins, but for all the reasons already noted don't tackle the complete sets.

When the silver Three Cent and Half Dime were replaced with the cupro-nickel coinage, the Philadelphia mint had a huge learning curve. The metal is hard, the coining presses were old, and the required striking pressure destroyed dies. For the Shield nickel, another issue was the cluttered design. The result was a lot of cracked dies, poorly struck coins, and frustration. For Philadelphia, however, it was a learning experience. The Liberty nickel was a much lower relief design, easier to strike, and wore much more evenly in circulation. Philadelphia also made all of the dies, and could replace dies easily. Philadelphia also had the lion's share of funding as the main mint, and upgraded its equipment.

Fast forward to 1912. Denver and San Francisco were tasked for the first time with striking cupro-nickel coinage with the Liberty nickel. The relative ease of striking that design was deceptive. The following year, the change to the much higher relief and much more cluttered and poorly-wearing Buffalo nickel overwhelmed the branch mints. They hated the design and experienced all the problems Philadelphia had worked through almost a half century earlier with the Shield nickel, but without the benefit of enough working dies, good striking presses, or adequate funding for upgrades. That's a key reason why the Philadelphia Buffalo nickels are much better struck than the Denver and San Francisco Buffalos. That, in turn, led to the less cluttered and slightly lower relief and much better-wearing Jefferson nickel designs.

So treat the Shield nickel as Philadelphia's high school metal shop learning curve project that paved the way for the designs that followed.

Very interesting!

Excellent!

I don't see why Dansco does that - who's going to crack out a $3000 proof coin and stick it in an album?

Popularity? Maybe it's just Twentieth Century (and Morgan dollar) bias. I focus on Liberty Seated dimes and quarters. The price to rarity comparison between popular Twentieth Century classic coins (no dead heads) is telling. PCGS coin facts estimates as few as 200 total survivors for some early key date Seated dimes. Any Twentieth Century classic coin of similar rarity (if there are any) would cost a relative fortune. In grades up to mid VF the Seated dimes can be had, if found, for under $1,000 to about $2,000. Not chump change, certainly, but imagine what a Mercury dime or Walker with only 200 survivors would be worth.
Really, it's a simple matter of supply and demand. There just aren't as many seated coin collectors seeking more than single type coins as there are collectors of the popular, and aesthetically beautiful Twentieth Century classics. I believe ultimately that is the answer to your question about Shield nickels. I have considered doing a set of Shield nickels myself, the series is not long and there are no branch mint rarities. The expensive 1879 and 1880 dates can be filled with more readily available proofs.
For what it's worth, I believe the Coin Facts survivor estimates are too low. But they do provide an order of magnitude for comparison to the Twentieth Century coins.
If you like them, go get those Shield nickels and have fun doing it.

I like Shield nickels. But so far as completing a nice set, it's out of my league, price wise.