Fm - perception they reward well and as there's nothing else to do with them easy to donate.
Dil - easy to come but, depending on the player, can be hard to get lots of and as it is needed for so many other things it's hit n' miss where anyone donates this and it can often be the last to be filled.
Doffs - these seem to be very hard to fill in my opinion if you don't have players that know how to come by them. Need to go out of your way to get them by assignments, converting, buying with ec etc. Some don't have space as whites aren't useful to them personally. They also don't pay that well it seems according to some. Donating method too is annoying but imagine they can't do select all as each doff is unique (relatively). It can often fall to those with the most resources to go to a fleet base and buy the doffs needed to get them underway of buy off the exchange.
Commodities - really this is so easy to donate but again it seems unusually hard to fill. It's a matter of using your own ec and going out of your way (potentially) to buy them and also the low reward for donating.
Exp - since we get so much of it or have a massive stockpile this gets filled first (yet gives the lowest return).
Summary - seems from my point of view doffs/commodities are the hardest to fill with dilithium somewhere reasonably hard too for similar reasons. Why donate 'your' resources when other can instead and you just do the easy stuff? That's not to say everyone has that mentality as some casual players don't have the means to build up enough resources to donate as well as investing in themselves.
They should consider dropping the cost at the base/increase the rewards for doffs and commodities although dilithium I'm not sure that will do much good, an increased reward might help.