Because they couldn't possibly put the a-grav units in swivel mounts. It's not as if a similar mechanism is already in use in all of the VTOL aircraft in the game.
And in that case the tank would fly rather than levitate.
I was referring to the sort of swiveling mount used, not the propulsion system. Though also note that, as gravitational fields are gradations of strength rather than a binary on/off setting, agrav would presumably behave the same way.
As far as i see it, there is two possible mechanisms behind the levitation.
1# A force that counteracts the planetary gravity. In which case the terrain wouldn't matter as the tank would basically be flying.
2# A force that pushes against the terrain underneath. This could use a swivel mount but taking the design of the tank in consideration I would say that a large part of the "wings" would contain the a-grav, possible multiple emitters that would exert various force to cope with variations in the terrain. In this case I doubt that the a-grav emitters or what ever function there is would be able to exert force at an angle capable of traversing such high degree slopes.
In case 1, it would still need to be directional. If it wasn't, everything that approached the magrider would be repelled. This obviously isn't the case.
In case 2, you're talking about presser beams, not agrav. Unless you mean to suggest that they're air-cushioned. I don't think you are, though, as it's plainly not the case. It's essentially a tossup between a localized, directional agrav field and multiple presser beams, though I think the former seems more likely, as the latter would be akin to attempting to "walk" the tank on invisible, single-jointed legs, which seems excessively awkward. Given the rounded nature of the magrider's hull and relatively low profile, it doesn't seem unreasonable that it can scale steep slopes. It wouldn't even be a matter of whether or not it has the force to do so, only at what angle the magrider's center of balance would be such that it would do a backflip off of the cliff. Add in the non-trivial forward thrust from the magboost, and...
Though from a gameplay perspective it should probably be toned down slightly, but still enough to be markedly better than other tanks at going up steep slopes.