One nice feature of the SCAR is the ability to quickly change barrels. The SCAR heavy has 13”, 16”, and 20” lengths available, depending on mission requirements.
20” is already very short in my book. I very much doubt that any tactical advantage of losing 7” of barrel makes up for the expense and logistics tree of having that option. And with any gas operated gun, barrel length changes affect operation. Yes ports can be changed, but that is a compromise, not an optimization.