Like most coins once ZEC reaches a certain level of mining popularity, you will need a certain level of hash rate to be consistently profitable mining solo. While over a long enough time-frame in theory each method (pool, solo) is equally profitable, the realities of finding a block within a usable time frame to offset expenses (ROI on rig hardware and electrical costs being the big two) make joining a pool a must for smaller miners.
You could get lucky and find a block day one, or unlucky and not find one for 6 months, all the while you are inuring the expenses of mining. So most people choose the pool for the more consistent and predictable payouts. Of course if you have a 200+ rig mining farm (1000+ GPUs), you may find solo mining more profitable as you do not need to pay pool fees. I just made that up and do not know the current solo versus pool breakeven point, but I am sure it is higher than most miners, myself included, can muster.