Why does the larger iPad pro support fewer LTE bands than the smaller one?
The larger iPad Pro was released before the smaller one, and presumably before the availability of a newer version of the radio used in the smaller iPad. Only an Apple employee could tell you the details of why the choice was made but I'm guessing this is indicative of a wholesale shift by Apple to use the more advanced radios in future products.

So to answer your question: the larger model wasn't "restricted" - it was released the year before the smaller model and it's likely the part wasn't available when it was being conceptualized and produced.