I think that data plans used to be expensive (I avoided getting a smartphone until 2014 because of the costs), but once you had a data plan for other reasons (e.g. being able to browse the web) then messaging apps were a cheaper way to send a message on the margin. My cheap phone plan (minutes only) with AT&T back in 2008-2011 charged me $0.25 per SMS sent and received, which added up fast with unsolicited messages, but eventually I found the option to just block all SMS to save money.