Well, I think it is more that many people that don't bother or don't know how than that cockers aren't clever enough. Cockers aren't collies, but if obedience training is made fun (which is should be), then cockers have fun too! Jesse loves it.
I know that Jesse is an American Cocker, but I guess a close enough relative to be representative here...
I have been doing competitive obedience with him from 8 weeks old... last year at 18 months old, he came second in Pre-Beginners in his first ever show and won out of Pre-Beginners in his third show. He was placed at almost every show in Beginners.
This year, we did three interclub matches in Beginners, we won one and came second in the other two... Two weeks ago, we had our first open show of the season, Jesse came 4th in Beginners and a placing in Novice is I think just a matter of time (he didn't do bad at all last time, but wasn't placed). I expect, or at least, hope, to win out of Beginners this season (meaning he needs to win the Beginners Class twice or win a Novice class).
Jesse always turns heads and gets loads of praise from judges. In fact, I think many judges are surprised to see how dedicated and focused Jesse is eventhough he is a spaniel. He certainly stands out amongst all the collies, but he is an excellent ambassador in the obedience circuit for all spaniels I think, particularly of course for the cockers and American cockers - which are indeed extremely rare in obedience! It's a shame as spaniels really are so able to do this.
Here's some piccies of the little fella:
Match in Culverstone earlier this year, we came 2nd (out of 4 dogs)
A 4th in Beginners in Ditton last year
Winning out of Pre-Beginners last year
These are mostly Jesse's rosettes and trophies, some also my saluki's Flash.