the first movie was more of an homage to hard boiled detective stories
there's quite a bit of investigating in the first one and those situations were able to foster some classic Carrey comedy.
The first movie features Ace as the main set piece, with his style of quips and mannerism permeating almost every scene.
The second movie had Africa be the set piece, rather than the character of Ace.
The recon rhino, the Waachu'tu trials, the shrine of Shikaka - these were more like stops on an African tour rather than an investigation.
But then I suppose that Ace's employment in the second movie was a ruse from the beginning.
In any case, Ace Ventura 2: When Nature Calls is the superior movie. Ace Ventura: Pet Detective is a better, more memorable story (and has a better Carrey performance).