They're not part of the C standard and they're not provided by all C libraries; we'd have to code our own and add thorough tests for the implementations (as we have for lgamma, gamma, erf, and other functions that may or may not exist in the platform libm). Making them optional is a no-go - you'd end up with code that works on some platforms but not on others. We've always provided fallback implementations in the past.

IMO this is the province of things like scipy.special. The Bessel functions (and many others) are already easily available to users of the scientific Python stack. I don't really see a need to have them in core Python.