There seem like two obvious reasons:
1. The team wanted to open up max contract space. Waiving and stretching Horford’s deal would have allowed that before resigning Smart, Richardson, Timelord, etc.; or
2. Management thought the team was close to being competitive, and assumed that Horford could help us advance deep into the playoffs, moreso than Kemba would have.
Based upon subsequent moves, Option 1 wasn’t the plan. Brad, et al, didn’t really believe in #2, did they? If so, why’d we hire a rookie coach?
The more I watch this season, the more it looks like we gave up a #1 (Sengun) for no good reason at all.