- Not tech-savvy and easily confused by keeping 6 different messengers around.
- People for whom multiple clients becomes a cognitive burden.
- Waste of storage space, memory and CPU power that those (hugely bloated, most of the time) clients incur, especially on mobile devices.
- People like me, with diminished executive functioning capacity, who are fucking tired of bad ergonomics of all those clients together and each individually, and who'd prefer a single, consistent, ergonomic and powerful UI to manage the torrent of messages they receive.
Storage space might be an issue on very low end phones but these days phones are starting with 128gb of storage which no IM app is close to filling. Mobile OSs are extremely good at sleeping apps and can receive messages while the app is not running at all using notification services. I would say that resource bloat is actually much more of an issue on desktop where you have 10 instances of chrome running and none can be closed while still receiving notifications.
The only thing I can think of is people who do not wish to have the app installed for privacy / open source reasons.