Because email was already standard when internet became commercial; if you wanted to be part of it, you had to have some email infrastructure set up. Today no

Because email was already standard when internet became commercial; if you wanted to be part of it, you had to have some email infrastructure set up. Today no company needs any IM, it's only a product (instead of a communication protocol) geared towards the masses. There's little business reason for them to use a standard protocol.