Primarily, Shaun, because Google couldn't give a **** about you, your privacy, or your data. As long as they make money off of you, they're happy.
The advantage of ProtonMail is that you can actually encrypt your e-mails, so that unless someone has physical access to the device you are logged in on, or the device your recipient is receiving from, no one can intercept your message.
The bridge is usable, and you will only have to use it once. After that, you shouldn't have to open it again. It just allows your preferred client to access your ProtonMail account.
I will go on the record here stating that the interface on the web is fully accessible. The iOS app has certain... quirks, but can still be used once you know what you're dealing with.
From what I've seen, Proton Support is very responsive. We just have to figure out how to communicate with them the right way.