There might be multiple reasons. This is very subjective I believe, but to me it's based on:
REST is the old and steady way. It is used by the most and provides an easy interface to API consumers. Because it's old (by no means a bad thing) most developers know of it and know how to consume it.
GraphQL is the new guy in town. It sure does save some performance (roundtrips and payload) for most systems, but does change the way we think of a backend. Instead of providing resource endpoints, it provides the graph of the data model and let the consumer decide what data to get.
As of the point of the "new guy", GraphQL is not as battle tested. It is new to most and there fore not adopted by others and the first-movers and startups, basically.
But again, this is a subjective question with a subjective answer. Currently I am using GraphQL for a startup to test it's durability and see if it can solve our needs. So far, it does by far. If you are to make a decision on wether to start a new project with REST or GraphQL you should be consider your needs and how much money/time you want to spend learning new vs. doing what you know and get to your goal faster.
