As a former Shotokan karate student
who has recently taken up Aikido (and loving It) I am surprised that sweeping the foot to unbalance an attacker, a technique basic to most Karate styles,is
not more widely practiced in Aikido. I
am aware that O'Sensei borrowed some tecniques from karate (e.g Mae Geri etc), but why not the foot sweep which
seems more Aikido than Karate anyway.? I woud be the first to admit my knowledge of both Shotokan and Aikido is sketchy,
but I would like to know more about this apparent omission.

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.

Foot Sweeps
 

In my training under Saotome and Ikeda Senseis we both used foot sweeps in technique but also did technique to defend against them.