Because it would make Sunny and Izzy's path to Zephyr Heights somehwat confusing, if the movie's director's word about the mountain they saw in the end of their duet song being Canterlot is canon. Sunny and Izzy would have to be looking at the Canterlot ruins from that mountain behind it, which would mean they more or less walked in an inverted S. Of course, they may have gotten lost along the way but it still is confusing. And an early Maretime Bay map shows the lighthouse being completely opposite Canterlogic. But it is concept art (Canterlogic itself looks wildly different), so I guess it doesn't count.