@gregdeth666Because the speeders are, by class type, just vehicles like the tank.
The only difference is that the driving soldier is physically present and can be damaged by blaster fire as well as force abilities like Anakin's magnet or the lightsaber throw. Same goes for the Wampa riders.
There's no way to ragdoll someone off because of the original balance thoughts (I guess). Like registering properly that the soldier is no longer on the vehicle, also vehicles despawn as soon as the driver leaves it. Means all speeder/light assault walker drivers would just instantly permanently lose their vehicle.
Sure, that issue is poorly solved in a disadvantaging way for the heroes, because unlike for tanks, you often can't just escape a speeder by running into cover.
Also speeders are in the air, while you can still dispatch a light assault walker of the Republic with some saber strikes comparably easy.
Imo it would have been better though if it was possible to push people off speeders, because that's simply what can happen if you drive too close to a force user.
Also hero gameplay is already more like Dark Souls instead casual, the speeder problem doesn't really help here.
However, speeders have been nerfed to the ground, are really bad to control now and have lost almost all of their potential to deal that much damage to anyone and very few people even play them anymore. So, I may guess that you have this problem on Endor specifically because of the lack of proper cover and big spaces?
Besides from HvH I mainly play Supremacy and my experience of getting harassed by speeders and vice versa ended abruptly when they released the speeder nerf within an update months ago.