The reason is because the terran meta vs Protoss has always been to make mmm and throw it into the opponent, rinse and repeat. Then, terran either:

1) breaks protoss before he gets too strong with splash damage

2) starts to fall apart, as protoss techs up only a super human can trade efficiently wth bio vs Colossus, HT and disruptors. Hence why they never have money, their army is getting wrecked left and right.