Usually because terrans at M1/GM level up to 6000MMR are rather dullards and fooled by Phoenixes think a good idea to attack and are warmly greeted by a Collo and Disruptors.
If they try to expand into third, Cobra expands in fourth and fifth.
No matter what the poor dunces do, there is no escape from the pesky Phoenixes that if need be kill even the Liberators while 6-8 Disruptors keep M-M-Ghost at arms length.
I feel sorry for all those poor terrans that have the bad luck to encounter him on Ladder.