If you play out the possible scenarios, you'll see why Lilo is right:
As Clemenza said, they were expecting (or at least guarding themselves against) an attack right at the point of picking up Michael. For all they knew, the Corleones might have sent someone of Michael's approximate size, dressed like him, to "do the job" on them right then and there. That's why they specified that the pickup point would be in front of Jack Dempsey's Restaurant--smack in the middle of Times Square, "the Crossroads of the World," the most crowded nighttime area of NYC.

If Michael had tried to take them out as soon as the car pulled up, he would have needed a machine gun--not exactly a concealable weapon--to nail them with certainty. Then he'd have to try to get away amid the crowds. If he had a handgun and tried to use it after getting in the car, he might miss, or fail to kill them immediately; or the car would careen out of control.

As Lilo said, he had only one shot at Sol and Mac. Getting them inside the restaurant, where they'd be far more relaxed, was the best shot.