Why Are Medieval Coins So Cheap?

I believe the answer is "Nope, not much interest" coupled with "too many specimens existing"

Ancients are an equal example, some are rare, but others are common and nearly worthless.

More the former than the latter, as far as I can tell.
Though it's kind of hard to estimate how many specimens there are when most never get out of local collections, and I suppose it also depends on what counts as "medieval".

Really, a lot differs from one series to another.
English medieval pennies are very popular but extremely common (meaning that the most common types are cheap but anything rarer gets expensive), same for Russian wire money (though it's slightly out of range for "medieval").
Most of Byzantine copper (at least the three big series: letter-denominated, anonymous, scyphate) is even more common, but much less popular. So they're pretty much all cheap.
Coins from the parts of medieval Europe that weren't Britain... vary too much, even as the general shape tends to be the same; the place was a patchwork of tiny states that all made their own currency, but most of that currency was some variation of "silver penny". You get a somewhat silly situation where the coins are kind of cheap because in general there's so many - and idenfication is a pain - but try looking for a specific type, and it becomes very hard (if more from lack of availability than any price problems).
Of course if you're searching for something from a more obscure region - good luck. And there are some other weird cases where a coin is a lot more expensive that you'd expect (coins of Kievan Rus, in particular, seem to be ludicrously expensive).

Early Islamic and medieval Indian coins are a challenge beyond most of us.

I've been really overwhelmed just with world coins in general. I have no idea where to start. With U.S. coins, it's a lot easier to know what's out there and what you want...

This, all the way.

That said, it depends on what you consider "expensive", too; with the latest exchange rates, my bar where I probably won't buy a coin is somewhere around $10, and other than the three Byzantine series I mentioned, hardly any medieval coins (at least, in anything remotely resembling idenfifiable condition) are cheaper.

I appreciate these coins; and I really appreciate that the smaller size stuff in circulated condition can remain affordable in the random occasions it does. The large, ornate silver coins in higher grades go for pretty big sums though. Researching these can be a challenge because IF they are in a reference book at all, there is still little info and it's likely not in English. For those who enjoy the thrill of the hunt and mysteries, it's a great area to delve into. But yeah, there is a limited supply of coins, a limited number of people who care to buy, and a very limited amount of info on these leaders who did little from states that lost their minimal relevance a looooooong time ago.

If we want to count 1582 Austrian States as still Medieval, I picked up a great silver coin with the leader in his ornate outfit holding a sword for under $15 a couple months back. I think only 3 or 4 people bid on it.