Maybe they wanted the cash in hand rather than a high variance company valuation. Or just wanted to transition their money and attention to something else. There are plenty of reasons to want to sell a company in a way that both sides come out ahead.

Also keep in mind that the public SO doesn't even have to change much for there to be "big changes". SO Jobs is where the money is made and they have private Q&A sites for businesses.

Perhaps they want liquidity, a more diverse portfolio, etc.

A bird in the hand is worth two in the bush.