Have to say that SITA is a traditional message exchanging platform and some carriers would like to reduce the workflow on SITA.
Frankly, it's for cost saving that using e-mail iso SITA. It's quite expensive for the big carriers to use SITA Telex coz the company will charge you every character. However, the SITA telex is still popular in the air industry that most of the ground handling agents, ATC, carriers, even authorities, e.g. customs are still reply on SITA telex. But I did see some ground handling agents don't have the SITA because they don't want to spend money on this, and they still do the bussiness very well. Probably few years later, the SITA message would not be mandatory for most messages. Or it could be a back up only, in event that the internet and e-mail is down.