Because a decade ago there was only Leiningen -- so all the books and tutorials (and videos) created in the early days of Clojure had to use it. At work, we started with lein (back in 2011 for our production code), then we switched completely to Boot in 2015, and then we switched completely to deps.edn in 2018. build.clj (tools.build) is a "recent" addition (less than two years since the very first 0.0.1 commit).

More and more introductory material is appearing these days featuring deps.edn but given there was a decade of Leiningen usage out there before deps.edn even appeared, the current state of affairs shouldn't surprise anyone.

As for more than one system, lots of languages have that situation: consider make -> ant -> maven -> gradle etc.

How do we ever move the world forward if we are only allowed to have one thing?