The polarizer needs to be able to rotate independently of other filters. Square/rectangular system filters only have one other filter type, the graduated/split filters, that needs to be rotated at all. The entire filter holder can be rotated to accommodate the angle required for the split/graduated filter. But what do you do when the horizon (or whatever line is dividing the split/grad filter use) is different from the angle you need to set the polarizer to?

Note that the square filter catalog includes a polarizer. If you don't need to use a split/grad filter (whether that's a neutral density, colour correction or colour effect filter), you can use a square polarizer anywhere in the filter stack. If you intend to use a split/grad, then you need to mount the polarizer in such a way that it can be rotated independently. In the Lee 100mm kit case, that means attaching a filter ring to the front of the holder (or the front of the hood). Why not use a special mount for the split/grad? Because you need to change the position of the rectangular split/grad filter as well as the angle, so it needs to use a regular slot in the holder.

(In the past, a square filter holder with a worm-gear-driven polarizer was available. That may have been for the similar Cokin system; it was a long time ago and my memory ain't what it used to be. In any casse, that could be put anywhere in the stack, but there was a problem: it took forever to adjust, and was quite fragile.)