My speculation is 'cause houses.. and idiot pedestrians. Plus it's relatively short... 82ave to 61ave... I think it's more lanes in lieu of a higher speed limit, but there's tons of places where the speed limit is artificially slow.

97St North of Henday, no reason that couldn't shouldn't be 100...
50St north of Valour Ave up to 259Ave is 50km/h whereas 50St south of Valour ave for all of 1300m before it dead ends is 80km/h and much rougher.