Why can’t I change warp speed when non-ion engines are active?

Well, if you go into the Tinker Panel and edit the hidden properties of the engine. You'll be able to set SupportWarpBurn to True and then that engine will be able to stay burning in time warp higher than 2x-4x time warp speed.

Someone who knows more about this than me will probably correct me if I'm wrong, but I believe it has to do with floating point accuracy just like the limit on physics distance. You can't time warp in atmosphere with engines active because of the lift/drag calculations, and you can't change time warp speed with engines active because the orbital equations are calculated based off time. Ion engines are the exception because the thrust is so low that it doesn't really matter

yeah, i noticed that too
however, in SR2 you can simply enable the ability for normal rocket engines to be usable in warp mode via XML editing