Network effects. I've been forced to install the client (Linux) twice for work. Worked fine for calls/chat, but I'm not keen on running it, to say the least.
Also, there are project managers in my industry who check availability of freelancers via Skype, rather than email. Remaining logged in keeps you visible with them. I don't do it, and I find it a shame that this is the state of affairs.
It's simple: Skype is big and everyone's using it. Kind of like Facebook.
I'll give you an example. When Skype goes down, nobody bats an eye. They just wait for it to start working again. When Tox (or to be more precise, qTox) behaves unexpectedly in a single situation, people come to me and say: "It's shit." (literally, no explanation whatsoever).
Since everyone's using it, they think of positively even when it behaves like a complete garbage.
It's the LinkedIn of IMs, huge in the mainstream business market. Most customers expect people to be reachable on Skype or Skype For Business (Lync). If you are a small business, considering you have to keep it on, might as well use it for internal comms as well.
