Real-time combat involves a lot of micro-management. Take Warcraft 3, for example. Your troops have a big array of special skills and attacks at their disposal, but in the heat of battle it's nigh impossible to use them with utmost efficiency, since the game turns into a mishmash. Turn-based combat, on the other hand, lets you isolate each enemy and attack it the way you see fit.
I suppose in the end, it all boils down to whether you prefer games that test your reflexes to those that involve more strategic planning. Not to say that Wacraft is lacking the latter, but eventually, the outcome is determined by how quick you are with the mouse and not by how well you planned your attack.