Different themes. Variation, just what we gamers really need.
Motorstorm was about mud and only mud. Clearly an "experiment" (not saying game's bad, I fucking LOVED the original MS).
Pacific Rift was genius, it had some mud elements but also added in fire, wind and water elemental which added great depth to the boosting mechanics.
Apocalypse takes this to the next step by taking what Pacific Rift did good and adding brilliant movie-like sequences that GREATLY enhances the gameplay quality since no lap will ever be the same (and this time it doesn't seem to be PR). Based on these videos, when a tall building collapses on the streets, you really need to adapt and take other routes. I love it and it's definitely not something we're used to seeing in racing games, let alone the Motorstorm franchise!
Clearly you've seen nothing of the game. This one has more strategy and track diversity than previous Motorstorms by FAR. Read what I wrote above.