I love how people keep bringing up their financial situation, of course they had problems maintaining a positive cashflow to pay for upkeep. That will always be a problem when you're running something that is illegal which also happens to cost money. Finding a good way to get people to pay you money for doing something illegal is very dangerous business and basically paints a HUGE target on your back.
Do you think Blizzard would let any private server live for months, even years, if they charged a subscription fee and reeked in hundreds of thousands of dollar on a monthly basis? They would shut that down so fast you wouldn't believe it, followed by a massive law-suit since now they've got credible data to prove a loss of income in a court of law.
So what do these small-time guys do? They could charge a minimal fee as low as a dollar a month to help with upkeep, again, painting a target on their back the size of a football stadium. Or they could going with something that is a bit more grey; optional donations for upkeep, trying their best to have a balance of both hiding said option to the general public to not raise to much attention, while still having it there for their loyal fanbase. See, there is a problem when you're trying to hide something, yet you want people to see it so they can help support your cause.
Secondly, how many do you think are interested in donating for a cause that might very well be gone the next day? Probably not that many, although that is just a guess from my part.
Third, let's flip your guys' argument around. Let's say Blizzard one day stopped accepting subscriptions and instead they put up an optional donation button on their website. Do you think they would retain even a fraction of their profits going from subscription to donations?(Talking subscriptions versus donations here, not cash-shop or other premium features) Of course they wouldn't! Does that mean that the product isn't worth people's money? Does that mean that if they would charge a sub fee for their game, that people would refuse to pay and forget about ever returning to Azeroth again? Of course not! Guess what; people rarely pay anything if they don't have to, people are cheap and opportunistic like that.
Look at Wikipedia, prime example. One of the largest and most visited websites in the world and guess what; they're having financial problems on a regular basis, barely making ends meet. Obviously a whole different scale here but the point still stands; if you don't have to pay for something, odds are; you won't.
So stop arguing that a private servers financial problems equates to there not being enough interest and that its not worth anyones time of day. Because with that logic we probably shouldn't have Wikipedia either.