The favoring of queen larvae from the successful sub-family is a separate, but related, question to why the workers don't produce drones of their own.

It seems likely to me that two forces are in play.

1. There is an extreme danger of some sort in drone laying workers. Probably some sort of in-breeding danger.
2. The advantage of cooperating with half sisters is in some mechanism that reinforces the genetic influence of the best sub-families.

Exactly what the danger is in #1, and the feedback mechanism in #2 is, remains to be explored by scientists.

Philosophically, I like the premise that is the queen who is pushing the sub-family advantage through the frequency of sub-family pheromones. If the workers are doing it, seems like it could set off an "arms race" within the hive that might threaten the very cooperation that is the basis of the honeybee's success.