Too many teams have a player that can contribute ~80% of what Rondo can to their squad -- and many of those teams have that player for less money. It makes more sense, then (for those teams), to go after other pieces than upgrading the point guard spot when chasing a title.
Yep, that's the truth of it. So the only teams who will give up a substantial package for Rondo would be the handful of teams with truly bad play at the point guard position who are otherwise ready to compete very soon AND any teams that are desperate enough to improve that they'd pay a significant amount for that extra 20%.
After all, in the post-season, an extra 20-25% from your point guard position can be the difference between advancing past the first round and getting sent home early. Indeed, with Rondo you might get an extra 30-40% in the playoffs.
But yeah, point guard is a deep position right now, so it's hard. On top of that, Rondo is an impending free agent and he's already 28 years old. So you're talking about a fairly specific set of circumstances that a team will have to be in to want to trade for him.
Of course, the reasons that D.o.s. points out are the same reasons why trading Rondo makes sense for the Celtics too, even if you set aside the uncertainty of Rondo's free agency. For a team that's still a couple core pieces away from being really good, would it make sense to pay 12-15 million a season for your point guard, when you could get 75-80% of that for half as much (or far less than half, if the replacement turns out to be somebody the team drafts this summer).