Sound currency arithmetic is a lot harder when you have to constantly watch out for the accidental introduction of a fractional part that the type system can't warn you about, and that can never be safe with IEEE 754 floats. (This doesn't just bite in and near finance: go use floating-point math to compute sales tax, you'll find out soon enough what I mean.)

Bigints solve that problem, but can't be natively represented by JSON, so there tends to be a lot of resistance to their use.