There is one reason why Microsoft might wait years, and only then sue a Linux distributor - precedence. It's the same principle patent trolls often use:
- they first attack the smaller companies that can't accept the risk of a lawsuit
- the smaller companies settle
- then the troll attacks a larger company and attempts to strengthen their position by pointing to the settling companies and saying "See all these people who settled because they thought there was something to our suit"
IANAL, but the United States legal system works on case law and precedent; each success at intimidating the small fish strengthens their position, especially if the small fish are forced as part of the settlement to acknowledge what Microsoft claims is their IP.