Comment by Kye

"Why didn't you use [proposed x]": high risk of seeming like the beginning of a tiring argument.

"I never thought about using [used y]": maybe not true, but sometimes it's more important to be humble than right. You invite them to explain why they used it, or ask what you would use instead.

The difference is the first comes with implications, while the latter beats implications over the head with a shovel and invites the askee to help you bury the body.