[Davetello]

The personalities were there, it's just that Fred Wolf exaggerated said personalities:

Leo became even more obsessed with leadership and more of a 'goody two shoes' than he was in Mirage.

Mikey's personality as the fun-loving turtle was exaggerated - he was a fairly serious character in the early Mirage issues (his one-shot aside) but he was the only option they had for the 'fun loving' turtle in FW.

Donnie is smart and good with his hands in Mirage, but he's not really played out as an ultra-genius in the early issues - he's more of a very clever tinkerer. In FW he is portrayed as someone who would be considered the world's greatest genius if he didn't have to hide from the outside world.

As well all know, FW radically changed Raphael. He went from a tortured soul to a wise-cracker who repeatedly broke the fourth wall. The original film might have confused us at the time, but thank goodness it remained relatively faithful to Mirage Raph.

Overall, when you think about the two different universes (one bleak and realistic and the other over-the-top and very light-hearted) it's no real surprise that the FW turtles are hyperbolic versions of the Mirage turtles, for the most part.