I think the best way to answer this question is to split it up into various categories of reasoning.
First, the amount of money this group spends is not really a lot of money. Raws for the older series are available in just about every P2P application out there. Therefore, evaluation of series usually do not cost us money except for the electricity bill to keep my laptop turned on to download things.
The series that we do choose to do, we will typically have DVDs imported, if available, in order to get the best raw material. As we do not work on very many series (due to the lack of manpower), the costs for these is not really too bad, and we can usually pay for this out of our own funds. Since none of our series are available in Blu-Ray (if I recall, importing the Card Captor Sakura Blu-Ray remasters is almost $1000), this is very easily handled.
Second, if people did donate, what would we do with it? We really don't need our own web server, we don't need our own tracker, and we simply don't work fast enough to need to spend several thousand dollars on raws at a time. To have an adequate server with ample resources to serve everyone is actually much more expensive than it looks. It also makes us more vulnerable if some organization hates us and decides to DDoS the server (I think by now, we've probably made a few enemies). For this reason, we host this on Blogger and our tracker is on NyaaTorrents.
The third reason is more of a philosophical reason. Historically, fansubbing is an almost selfless endeavor. We are here to provide a service. Back in the days of VHS fansubbing, the only fee you were supposed to charge is a fee to cover the duplication and shipping of tapes. The Technogirls did experiment with people sponsoring episodes, but that didn't really pan out. Now, in the days of digisubbing, technically we are supposed to charge nothing (and people who "sell out" usually are ostracized in the fansubbing community). Fansubbing is supposed to be an almost selfless, free service, done from desire and passion for the anime you are working on.
(As a side note, Google Adsense is turned off here. It better stay off.)