The timing of the suspension is difficult for the Leafs, because they already have multiple D facing injuries. Hence, Lajoie getting a call-up.
They get a bit of a break with it only being 5 games. They have a Western road trip coming up that has 3 games in 4 days, including a back-to-back. They will only lose Reilly for the first game of that road trip against the Coyotes. A 6 or 7 game suspension would have put them in a very difficult position.
It was a non-hockey play when the game was as close to over as you can get. That's why he got 5 games. The reason it wasn't longer might have been his lack of previous discipline, and the crosscheck not being directly aimed at the head. It rode up from the shoulders to the face. Had it been directly to the head, I'd imagine he might have received a few more games. Ultimately, they don't want players thinking they can go after other players for retribution in non-hockey plays.