Because Barcelona is a more successful tournament for him, that's why. The place in the calendar may not be ideal but it's like Fed skipping Basel to take a break. Nadal can easily rack up the points in Barcelona. If you're also aware of the 4 500 rule you won't be saying I don't make sense. It's you who doesn't make sense.
Tell me, where can Nadal play 4 ATP 500 during the year and expect a sure-fire win.
I can almost guarantee you that if Basel was the 4th tournament in a 6 week period where Fed was playing 3 other AMS 1000 events he wouldn't play Basel. Djokovic, albeit due to a death in the family, withdrew from Serbia next week and his parents OWN the tournament.
I get that Nadal relishes the fact that he dominates Barcelona, its a good tournament with a lot of tradition and obviously its in his home nation but you have to look at the big picture. If Nadal was scot free, never got hurt and didn't have lingering knee problems which date back to 5 years ago I would say that its ok to play this tournament. But this very workload cost him a Grand Slam title in 2009 when this huge work load did take its toll and he got hurt.