it might have to do with Aikido's emphasis on multiple attackers. one never knows if there is gonna be more than one attacker. so sweeping would put u at risk of losing your balance. lifting your foot to sweep would weaken u grounding connection with good olde mother earth. it hard to maintain your center when only one foot is on the ground. and being centered is essential to Aikido.
but i would never tell anyone not to sweep. it certainly has its applications ans situations where it might be warranted. it just yet another tool which you can use to defend yourself or another.