The PEP has some notes on this.

https://peps.python.org/pep-0810/#global-lazy-imports-contro...

https://peps.python.org/pep-0810/#making-the-new-behavior-th...

the main reason is import-time side effects, particularly intentional ones like the registry pattern.

also let me plug my work project, an analyser which will scan your code and see where it can safely enable lazy imports (tagged alpha, but we did our best to get a usable release out before 3.15 landed): https://github.com/Facebook/lifeguard

Completely nondeterministic import order affected by code execution patterns?