The MSP430 microcontroller series has a few features that make it popular in intermittent/batteryless applications:
Widely used in both batteryless and standard applications, which makes finding tools and support relatively easy
Low power draw with configurable low power modes, important in the low power environments of most intermittent devices
Some MSP430 boards come with FRAM, non-volatile storage that is critical to maintaining state and data through the regular power losses of intermittent devices.