Mainly because it is ten times more humerous than Tropius and is way better in direct combat. Gen 1 Exeg was a monster (Baring Blue's rather dillard version with 3 moves) and it could easily stomp most stuff flat (lol). And its prevo is creepy as hell :)
If so, Tropius is my vote. Look at it, it's a goddamn banana dinosaur? Who doesn't love dinosaurs? Not any kid that I know, that's for sure. Now look at Exeggutor. What is it? Some silly, creepy coconut tree. Who wants that?
The problem with Tropius is it looks cool and bad-ass, but it sucks horribly bad. I remember catching it for the first time in Sapphire and then looking at its stats...most disappointing pokemon of all time.
Exeggutor on the other hand...the thing is an f'in boss.
For people that say pokemon past the 1st generation don't look like pokemon anymore, one of my first examples to the is Tropius, it's cool, unique, serves a purpose in feeding other pokemon, what else do you want? [shut up about stats]