Definitely not as new as it could be.

When email came out in the '80's it was the first time for so many people that typed office correspondence was not produced by a professional typist.

Expectations were not that high from that point, then one day the internet came along and way more non-typists started typing, plus students got their hands on cellphones and started silently texting each other at high speed when they couldn't talk during class.

The whole time, things like Microsoft Word by the early '90's were fully capable of quite good layout and formatting. Pretty much regarded as formalities intended for professional use though, plus Word cost money and nobody could expect plain text to compare so why bother starting to seriously capitalize and punctuate now?

Plus on low-resolution monitors, it was sometimes more difficult to read any of the word-processor renditions until you printed them, so plain text it was, non-professionally typed for everything but the most formal true documents.

Anything other than lower-case has always been forced.

I guess the short answer would be, "Sir, this is an internet."