Plenty of new research languages are coming out with linear types.

Building a mainstream language is extremely expensive these days, to the point that probably only a handful of companies can attempt it. I work in Scala which teeters on the edge of mainstream, and it's not the language itself that needs major backing, it's all the other stuff - IDEs, library repositories, build tools, profilers. I would love to leap into e.g. Idris, which does have linear types, but who's going to write an IDE for it? Even Rust struggles with this.

And it's very hard to retrofit linear types onto an existing language, if you can even get a quorum in favour of doing so. Haskell is trying, and I wish them well, but it's not going to be easy, and again they're barely mainstream.