The tug is automatically selected base on airplane weight and in some cases, the airplane height.
So yes, if your question was "why I don't get the big Douglas on my 737", it's because it's just too big for it, the cabin won't fit under the nose, it's just a model that can't be used on a 737 so, for the time being, enjoy a selection of several types that all have a Towbar.
As discussed in other threads, we already had a smaller Towbarless truck in FSX and P3D, but we didn't want to take the cheap way out and just port it, we want to remodel it from scratch, to do it right.