This has nothing to do with poor planning or not having the money to buy tickets early. It has everything to do with past experience and how Coachella has changed for the worse. In the last 10 years i've attended, the festival has never sold out like this. But I always knew it was a possibility and if you are surprised you should be surprised that it hasn't happened sooner.
Coachella used to be a festival that would release a line up and sell tickets for individual days and the days wouldn't sell out till the very last minute (for the most part). The beauty of that was it gave the fans time to go through all the bands new and old and figure out a plan of what you wanted to spend your time and money on. It gave you time to get your friends together, secure a house, RV, hotel or simply camp.
Since Coachella took away that option things have changed drastically. The introduction of "3 day passes only" is where Goldenvoice fucked up. In doing that, the festival lost its roots!! When they start booking acts such as Jay-Z, Kanye, King of Leon etc there is only one option left. Jump ship!! Its time to leave Coachella in the past and enjoy the good memories of the old school days at the Empire Polo Field.
You broke my heart Goldenvoice. I will never forgive you.