Well, I would guess first and foremost: why change? People that already use putty probably have no desire to switch just to switch. If the alternative isn't SIGNIFICANTLY better, most people aren't going to switch just to switch.

Second: not everyone is on Windows 10 which is a requirement for the Linux subsystem.

I guess last but not least: in both cases they require installing software. Some corporate users have no control over their own laptops. Putty has a portable version that you don't have to install anything to use.