You mean besides being the most durable tank in the game? The TTK in 1v1 is the reason it got hit with the nerf bat. It may not do as well against Elephant and JT but they were undefeatable in 1s when you got more than 1 of them.
When was the Churchill a meta go to tank in 1v1? Must have been years ago after UKF release because I really don't remember Churchills being meta in 1v1 in any tourney I ever watched. I honestly can't even remember a game in 1v1 where a Churchill was even build.
I personally can't say I ever found Churchills to be particularly OP in 1v1 or 2v2. Pretty sure the nerf was for 3v3 and 4v4 and not the smaller game modes.
It's also far from being the most durable tank when there is the 1280HP 375 armour KT around.