Because treads are worse than tires for compaction overall. As a soils expert explained to me (phd in the subject), compaction is a function of weight, but the function is something like 0.2*(weight) (obviously it is more complex than that and depends on the exact soil type). Compaction happens only where the tires/treads touch the ground, and tracks touch a lot more ground when you turn so even though there is less damage across the field you lose all of that and more when you turn around. Not to mention tracks have to slide sideways to turn and that is bad for your topsoil.
> Not to mention tracks have to slide sideways to turn and that is bad for your topsoil.
It's unusual and perhaps not suited to large tracked vehicles, but there is an alternative to using differential track speed for turning: track warping. Basically the vehicle bends the tracks one way or the other to induce a turn. The turning radius sucks though.
