Why use <stdbool.h> instead of _Bool?

They are same. bool is an alias for _Bool. Before C99 we used we dont have this type. (Earlier the use was limited to an integer tyoe with 0 as false and 1 as true).

You may not use it. Even you can undef bool (but it is recommended not to do so). But including it (stdbool.h and bool alias of _Bool) is good because then if someday it becomes reserved your code complies to that.¹

_{1. You can use bool other way but it is better not to. Because in general when this stdbool.h is introduced it bears the plan of gradually making it standard and then even more stricter rule applies where we can't use bool as something other and it will be reserved as keyword.}

The common practice has always been to use bool but when the type was officially introduced into the standard in C99, they didn't want to break the "roll-your-own" implementations. So they made the type _Bool as kind of a hack around the unofficial bools. Now there's no type name collision. Anyway, point is, use bool unless a legacy codebase breaks.

The obvious type to add into the language was bool. But unfortunately, plenty of code was written that included bool in other shapes and forms. Recall that support for a boolean type was added only in C99.

So the C language committee had no choice but to pull out a reserved identifier for it (_Bool). But, since the obvious choice of type name is still the same, stdbool.h was added to allow users the obvious name. That way, if your code didn't have a home-brewed bool, you could use the built in one.

So do indeed use stdbool.h if you aren't bound to some existing home-brewed bool. It will be the standard type, with all the benefits that type brings in.