"In February 1998, Ericsson Radio Systems banned the in-house use of Erlang for new products, citing a preference for non-proprietary languages. The ban caused Armstrong and others to make plans to leave Ericsson. In March 1998 Ericsson announced the AXD301 switch, containing over a million lines of Erlang and reported to achieve a high availability of nine "9"s. In December 1998, the implementation of Erlang was open-sourced and most of the Erlang team resigned to form a new company Bluetail AB. Ericsson eventually relaxed the ban and re-hired Armstrong in 2004."

Not wanting to rely on a fairly esoteric in-house language makes some sense.

Since then things have changed significantly of course.