Because with the larger field, the deeper end zones, and the separation between offensive and defensive lines football in Canada is even more of a passing game than in the U.S. Passing is not Tebow's strong suit.
His passing has vastly improved. Watching him now compared to his first stint is totally different. If you don’t see that you never played much football.
I would guess that with the lower overall talent level in the CFL, Tebow should be able to play there. While passing may not be his strong suit, it should be strong enough to allow him to play there (and it needs to become a strong suit.) He needs reps in game situations and he needs practice maintaining his improved mechanics while under duress. The CFL could provide that.
A few years ago he was the starter on an NFL playoff team, although I will grant that he was a poor man's Trent Dilfer at the time. He has improved his throwing motion rather dramatically since that time, but he has had very little game action.
I don't buy the persecution angle, but I think he's a lot closer to being good enough than he was when he was actually playing. If he wants to keep trying, Canada could be a good option.