If games like Fallout 2 are so great, why did they disappear?
I know the question is a joke, but I'll try to answer in a serious manner.
First it's not so easy to make a Fallout clone. Many tried and failed (Metalheart, Goldenland, Omega Syndrome, Krai Mira). There are thousands of parts that have to click in order for such an effort to succeed.
Besides it's easier and cheaper to make Diablo clones (of which few are also successful, but that's a topic for a different conversation). So investment goes in this direction.
Fortunately the experience gained from those failures, Kickstarter money and the ressurgence of oldschool gaming allowed for the subgenre of the "Fallout-like" to reappear. We've got Underrail (quite different but with lots of similarities), Atom, soon Encased will join. And hopefully more in the future.