It's the fact that they don't necessarily have 'vendor lock in' - and substitution is easy. And it's an easy thing to copy.
It's designed for small teams, and small teams can switch to the 'next, cool thing' in a heartbeat.
At the end of the day it's just chat. That's it. There's nary anything special about it. They did a good job of it.
That said, brands to have sticking power, and if there is no reason to change then why?
They have some new MS execs, who might teach them the ways of 'sinking their claws in' :) and becoming incumbent.
I should add, the larger the company (even with small teams), the bigger the switching costs. After about 500 people, IT kind of becomes 'detached' from the people, and doing anything on one's own becomes harder, so tech becomes entrenched.
This company is the next 'IPO pony' the VC's are building up, hoping to push onto markets at high valuation to get a big bang.