Tanks may be faster on flat terrain and usually better armored, but mecha are more mobile and quite usually better armed. Add that to the fact that the Junkyard is very often [i]not[/i] flat terrain, which makes the mecha a much more versatile choice. Note, this does not mean that conventional weapons like tanks are unused, and that's why if you wish to use one, you should be very aware that you can be outmaneuvered by people using said mecha. If you still wish to do as such, I would suggest scaling it up in order to compensate for such things.