Would the judges know how the votes were going? That Beth was in the lead? If it was obvious Matt wasn't going to catch up then I don't see the point in bigging him up with all the 10's - it will build his hopes up more and be more of a crashing disappointment when he doesn't win
And humiliating for him - someone scoring eight 10's loses to someone that didn't get any
Giving him all 10's (and Beth none at all) just made it look like blatant favouritism and could have put people off voting for Matt and go for Beth instead - so the judges shot themselves in the foot