Trucks were more subject to differences in terrain than half tracks. They'd run a lot faster on roads, but a lot slower cross-country (maybe even stop when it gets real mushy). Currently all tracked vehicles run at full road speed whereever they are - except when driving on a gradient.
To make trucks worth while you'd need to slow all GVs down by varying amounts depending on terrain - but keep the speed high on roads. That means a lot of modelling, changing of spawn points (approx. 5 minutes from enemy bases) etc.
It would make driving along roads a bit more of a challenge - but you'd also need more roads!
I would love to have more realistic vehicle speeds and terrain, but to be fair with HTC it's simply far too much work to achieve.