They piggy backed on rapidly growing XMPP and then became lazy with keeping compatible with the rest of the xmpp federation and at some point the s2s connection stopped being feasible as they never implemented TLS for it, and did't really care as most xmpp users were on their server anyways and thus did't use the s2s connection.

Its not a typical nefarious EEE story, but it did a lot of damage to the xmpp federation anyways.