Having enough intelligence to be able to build simple contraptions and having enough intelligence or sentience to have full command of the Force is two different things. A predator in the wild knows how to set traps for its prey, but that doesn't mean that if it broke in to my house it would be intelligent enough to turn on my computer and send email. Ewoks have primitive intelligence that allows them simple thought. My guess (btw, this is just my opinion, since my word is by no means 'canon') is you need much more than that to control the Force.
As for wookiees, they're supposed to be intelligent beings. Don't forget they're great with mechanical devices and their technological abilities are quite high. Yes, they live in the trees, but unlike ewoks and lesser "creatures", wookiees are considered quite intelligent and thus why it's not unfounded to have a Force strong wookiee.