The way I see it, the manufacturing company successfully developed the 400-rated fusion engine, then had to create a BattleMech to carry it. They went cheap on that part, hoping the SLDF wouldn't care in light of having such a fast 80-tonner, and perhaps they'd even pay to upgrade the design.
Unfortunately, the SLDF did care, and dropped the Charger as soon as they could. Actually, do we know if the developing company was in Hegemony territory or not? That might affect what technology they could get for the -1A1 model...
Edit: Ah, it was developed on Luthien. Yeah, given how tight-fisted the Hegemony was with their tech, it makes perfect sense that standard tech was used on a BattleMech developed in the Combine...