Question about reasons behind Thunderbird

[Martin Sjöstrand]

How come Thunderbird is the standard email-client? Not that it's bad,
but I recently switched to Evolution, and I find that it fits better in
Xubuntu theme-wise than Thunderbird. And in my own humble opinion I
also find it easier to get an overview in Evolution than in
Thunderbird. 
I'm not suggesting that Xubuntu should come with Evolution, I'm just
curious of the reasons behind using Thunderbird.

Regards

Martin