I am a season ticket holder for the Pacers and watch NBA at home or in person almost every night during the season. With a few exceptions, the base level of defense in the NBA is actually very good (a bad NBA defense would probably completely shut down a top tier college offense). A ton of time and effort goes into communication and overall strategy, but the cards are just very much stacked against the defense in the NBA for a few reasons.
1) The offensive schemes are amazing, and the teams hunt mismatches/advantages ruthlessly. (Sidenote - Not sure why the mismatch hunting isn't more aggressively pursued in the college game...)
2) The offensive players are orders of magnitude better than college - shot making, movement, making reads, very nuanced movements (ex. 7-foot centers stepping out and consistently nailing 3s is hard to stop, and unclogs the lane for more effective penetration.)
3) the schedule density limits how much game-to-game defensive game planning can occur. Not much you can do on back-to-backs except line em up and react, whereas the offense dictates the action.
4) Refereeing in the NBA is much more of an offensive lean (for everyone not named the Thunder, that is). Almost impossible to get a charge call most of the time.
5) Did I mention the shot making?! LOL This point cannot be understated. The stark contrast honestly makes college basketball very difficult to watch at times.