True bipedalism is pretty rare. Frankly, toddling around half-unbalanced is a good way to get eaten in the wild.

In all seriousness, in a tetrapod body plan (four limbs), to get bipedalism, one needs an adaptive change for the other two limbs. If the two front limbs aren't doing anything useful, generally speaking, evolution favors keeping four legs, rather than withering perfectly useful limbs. Obviously, that's a generalized statement, but let's talk about specialized forelimbs.

In humans, they are for fine manipulation. This is also true in all the great apes, bears, raccoons, otters, and the like. Hominins are the only ones, apparently, to get full bipedalism for this reason, and that is likely because we were the only ones with the right evolutionary pressure.

In badgers and pangolins, it's digging. (Badgers have only partial bipedalism.)

In birds, it's flight. (Yeah, all birds are bipedal!)

In penguins, their wings adapted for swimming control.

For other flightless birds (emus, cassowaries, etc.), current thinking is that they first evolved flight, then evolved to no longer have flight (say, when evolutionary pressures and genetic happenstance favored them being big enough to fight back, instead of flying away).

Now, imagine the evolutionary pressures that led to snakes losing all four limbs!

So, basically, the four-legged form just needs a genetic mutation and a complementary evolutionary pressure to encourage bipedalism, and there just aren't very many reasons to pressure for full bipedalism.