I think it's not unreasonable to expect there to be written dialects as well as spoken ones. A comparatively huge amount of a person's communication is written these days, where previously it would have been face-to-face or with a phone call.

The general spoken register where I live skews informal, so I assume this has carried over to written communication. Using capitals and punctuation comes across to me as stilted and like an email or a letter rather than a text message.

It's an interesting subject definitely