Simple answer: Cost.

When debugging you'd want as much info as possible and you'd want to be able to simply tail + grep it. I've been told to log less because the amount I was logging would burn a hole in the pocket when deployed to production. Sometimes, at scale people only send WARN (maybe even only ERROR) and above in production which is sometimes not enough when trying to debug a system on fire.

While ELK does a great job of indexing the contents, and if you depend on it for BI, you should definitely still keep it for use-cases where just select+grep won't suffice. But for just storing logs, and being able to select, stream and grep laaarge quantities of logs, Loki will come in handy.