Solely because they got gutted to accommodate their shit awful design decision to make bolster a global. I recall vividly when tommies were just straight up better grens at 4 men and monsters at 4. Now they are a joke as 4 men and alright with 5. That's exactly the point in trying to make. Either 4 men are trash so 5 men are balanced or 4 men are worth the cost and 5 man tips them way over the edge. You can't reasonably balance such a power spike. Cons extra model either comes from a full tech or 3 combined side techs and 2 buildings THEN takes a munitions cost AND their only weapon slot and it still had to be toned down by removing the cover bonus and lowering the reinforcement bonus. Increasing model count is such a colossal upgrade its hard to balance and has caused nothing but problems balancing tommies from the word go.
Both 4 and 5 mean are fine. It’s an expensive upgrade that lets them scale into the med game and then double BRENs let them scale into the late game.
All this is necessitated thanks to their laughable veterancy bonuses.