the main reasons for my preference are that: gmail/fastmail are simple, I can use from multiple computers easily, have good defaults and most importantly, they almost always work. Outlook users seem to constantly have something broken.
I do realise that i am reaping the benefits of a centralised system, and in the case of gmail, monetised by advertising, surveillance and user lock-in. Probably the ideal for me would would be a self-hosted web client, e.g. Mailcow