Why should Fastmail invest in Thunderbird, an open-source email client run by a completely unrelated organization? Of course they could, out of good will. But you can't demand that they do.

This is just their webapp wrapped in an off-the-shelf browser engine. Hardly any development resources needed. It could have been quickly put together to tick a checkbox for some big client, the revenue from whom could help them work on features that actually matter more. None of us needs to use it. But somebody must have needed it, so here it is.