Exactly. See also statistics for "how many people are left-handed". The incidence of left-handedness didn't go up; acceptance of it went up, and people stopped trying to force other people to hide/mask it.

It is also possible that depression itself is more widespread. But it seems exceedingly likely that acceptance, willingness to talk about it, and general understanding that the correct answer to "chemical imbalance in the brain" is not "cheer up!", have improved substantially.