Many reasons. Cost being the biggest (64bit dies would cost way more than the 32bit ones) and power consumption (having a complex pipeline with 64bit registers

Many reasons. Cost being the biggest (64bit dies would cost way more than the 32bit ones) and power consumption (having a complex pipeline with 64bit registers isn't great when you have to run on a coin cell for a year). Similar reasons to why 8 and 16 bit micros stuck around for so long in low cost and low power devices before Cortex M0+ became cheap and frugal enough.