Not certain this is the reason why, but there is a real perf hit for full disk encryption which not everyone needs to take, especially for devices without hardware accelerated crypto. Lower end devices can slow to a crawl.
Any modern system(even on the low end) that you could conceivably want to trust important data to can handle encryption requirements with ease.
One problem if we are talking about Bitlocker specifically is that if the drive reports that it supports encryption, then Bitlocker offloads the responsibility to the drive. And the drive encryption might be badly implemented.