Why is "stackexchange" sometimes abbreviated "sx" and not "se"?

I've been posting a few questions on the TeX site lately, and have come across several comments on questions saying Welcome to TeX.sx.

Why is stackexchange abbreviated as sx? The few times I used an abbreviation myself, it was se.

Phonetics

Because the letter x is pronounced /ɛks/, which is also how the beginning of the word Exchange sounds. There is no phonetic difference between the name of the letter x and the syllable ex.

Janus Bahs Jacquet is correct, people use the "x" because it sounds like "Ex" when you say it.

I can't think of any other reason why people would use "x" here.

I've never seen "SX" used

This may be because you and me browse different communities, but I've always seen "SE" used:

English.SE
Gaming.SE
SciFi.SE

Some others never get "SE" or "SX", like Stack Overflow = SO (note: Stack Overflow has existed for longer than Stack Exchange, so it's normal that it uses a pre-SE nickname)

But like I said, maybe the places you frequent do use "SX".