There might be another reason but this is what I understand:

Beacons are a type of "contract" that tells the Neural Network to track statistics for a user. Each user tracked adds additional calculations and increases the total size when making a superblock. If there were no expiration on these contracts, the amount of users being tracked but no longer crunch can build up and this can cause issues with superblocks eventually.

The solution to this problem was to have beacons expire after an amount of time. Issue is, if your beacon expires before you stake all Research Rewards being tracked are lost. What @Ravonn proposes is a system where your old Research Rewards can be forwarded to a new beacon once your old one expires. That way no rewards are lost.