First of all: donations bring in very little money. It's like trying to convince a software pirate to buy the software he just downloaded for free. Very, very few do it. Plus everyone thinks there are enough others doing it. So, there you go.
Commercial products are much more effective to cover costs and to subsidize the development of free products. And the users actually gets something valuable in return. Win-win.
In addition, it simply wouldn't be right to sell commercial products and then also accepting donations at the same time. At least not without being fully transparent about what the donations are being used for.
There's also the question of fairness: if the donations do cover all of the running costs, how is the extra money used? How should we split donations among contributors and library authors fairly? How do we provide enough transparency about how high the running costs are and how much of it has been covered through donations?
Simply put: we'd rather have you donate to other library authors who need donations.