Lawyers charge a lot per hour but also have large overheads, especially trial lawyers. The amounts mentioned in the fundraising page seem small. I am guessing Chelsea Manning already gets a discount and that they will have to raise much more later on.
A high profile criminal case such as the one against Chelsea Manning where the government throws in many man-years will only be successful if the defense also throws in a team of competent defense attorneys, researchers, assistants etc. In a huge case like this that stretches over several years and where there is vast information to deal with, this could amount to decades of man-years.
A one man boutiques usually can't handle such cases and the largest law firms typically don't take such cases. So the defendant often has to pick among law firms with 5-20 lawyers.
Such small firms usually cannot afford to allocate that much manpower to pro bono cases.
But even if they could afford it, there are numerous faster and cheaper options for marketing. As for professional satisfaction, there are plenty of cases that provide that and pays a salary meanwhile.