AIM, Yahoo, MSN etc. were mostly plain text systems, built around presence and a single client. Then came mobile phones, with unreliable connections, with messengers which allowed integrating pictures etc. and easy sign up (iMessage by using apple ID, which every iPhone user has; Whatsapp by using phone number which directly linked the contacts), which worked without battery draining connection.

Skype originally worked by making random clients "super nodes" which coordinated the network, without needing a big data center managing it all. Making phones super nodes wasn't an option so they had to change their protocol in big ways.

So adapting was a cost and changed user experience, while newcomers grew.

In case of Google there was the addition of missing strategic leadership, where each team built their own new messenger, but nobody maintained any old one.