Even a minor impact or knock can put a ding in an alloy wheel, even landing a wheelie a little hard, bumping up a kerb or a fairly minor pothole. Also you can't keep getting them straigtened for ever as the metal can fatigue. A decent outfit should be able to let you know if yours can be bent back into shape again. If they straightened it properly the damage shouldn't come back – although it's possible they never got it quite straight.
The best option, and the most expensive one, is to try and track down a pair of good condition used wheels.
I buckled a wheel on my Kawasaki ER6N last year when I ran over a large hole in the road. I had it straightened and re-balanced by a specialist and it been fine. I recently read that a buckled wheel can be dangerous. I buckled the front wheel twice, the last time a month ago - different locations - both times the front wheel. The thing is I haven't done anything since the last time which would have damaged it so could this be the original buckle coming back in some way?