Primarily it's the fact that it's basically a platform for the Prism Cannon, which is a single shot weapon fired by a pilot with a BS of 3. That means you have a 50% chance of hitting. That is very unreliable. As a Guard player, the fact that our main battle tanks, while a bit more expensive points wise, don't have to roll against BS, make them a bit more reliable (Depending on your relationship with the scatter die, of course).
You're paying 120-150 points for a weapon with 50% reliability? Not gonna happen. You can spend those points on two kitted vypers with Brightlances and get more reliability against vehicles, single large targets, and most other targets you'd be firing the Prism Cannon against.
Of course, the Prism has more uses than just a platform for the Prism Cannon. Sadly, it is more difficult to see, so therefore most people go for other things that can perform just as well in that regard, such as a Falcon.