Because most of them are open on Saturdays.

I have noticed that they are usually closed on Monday to give them two days off in a row since they usually work on Saturdays. But maybe some prefer a mid week break.

My guess is that Wednesday is a slow day for most salons and they probably don’t do enough business to justify the cost of being open, so they close their doors as a cost-cutting measure.

@syz is correct. Historically, hair salons were open on Saturday to deal with the women who were working all week. But the hairdressers didn’t want to work 6 days a week (and it probably wasn’t legal) so over the years, salon owners gave them Wednesday as a closed day.

Of course, the whole industry has changed in the last 50 years, but the tradition continues.

In the small city near the small town where I grew up, many of the businesses were closed on Wednesdays in order to be open on Saturdays.