Because switching a mail client that has a very specific workflow and set of features is a huge shift and you have to change a ton of habits and relearn muscle memory. It's worse than switching browsers because browsers don't embody as much workflow (and tend to be fairly similar to each other)
Been there with Google Inbox. I want one client that is going to stick around and works on all my devices.
I feel that with Inbox... still miss it. That said, if something comes along that is as good, I'll still switch back. (And maybe Big Mail will do that?)
In general you might because of sinking the purchase cost, but in this case it's a subscription, so all I (not GP) can think of is the inertia and inconvenience of switching mail client if you only really wanted to switch (or add an) OS.
Been there with Google Inbox. I want one client that is going to stick around and works on all my devices.