What are some forks of Bitcoin that ultimately failed? Being as many of the popular cryptos today were forked from Bitcoin at some point, I'm curious which past forks seemed promising, but ultimately failed at some point..

Sort of a history lesson if you will, so what past coins failed and why?
It kinda depends on your definition of "failed".
See http://mapofcoins.com/bitcoin for a list of maybe not the whole tree, but a fairly large one. You can select "defunct" coins at the top, to mark those. Click on the symbols at the right to get info on that coin.

Now, defunct on that site seems a hard bar to reach, despite the high number of those. For just "failed", you can probably add a whole lot more,

Maybe the most notable failed/defunct one (not because it's known, but by its descendents) could be Tenebrix, which begat Litecoin.