The simple answer is "to cash out." Think of it this way... you have a small business that has been growing successfully, but the space is incredibly crowded and has giants like Starbucks, Peet's, Keurig, etc.
You're still on track for great growth, but there's a big risk you could stagnate and die off. Then on the other hand you have these investors coming who want to throw large sums of money at you. And since the coffee business isn't necessarily like the technology startup space where a lot of your employees have equity, there's a decent chance you're keeping a lot of that.
So when you're looking at the possibility of banking 10's or 100's of millions now, vs. a gradual payoff for a business with steep competition that adds a massive amount of stress to your life, for some people the answer is quite simple.
Of course some are very passionate about the business to the point where they can't see themselves doing anything else, and they might not sell, or might not sell the entire stake. But you honestly can't fault someone for taking a guaranteed payout after busting their hump on a business that, statistically, should have failed (and still might).
Big outfits continue to acquire and... "beat down" the better shops, in our area. Make money, I guess -- but the product invariably seems to suffer.