I can guarantee Bill's reason is because he wants the money he provides to be used as efficiently as possible. You want the most benefit to be provided for your dollar. You want to donate to the organization who can use your funds at 70% efficiency, and not one that will use it at 7% efficiency, otherwise you are providing less good for your dollar.

Based on his other comments, I assume he was referring to the challenges of finding the right places to spend money--places where the money will have the most positive effect. It's easy to give money away. Goodness knows there are endless people and organizations who would gladly take it. What's much harder is knowing ahead of time what impact a given donation will have. Or, to put it differently, knowing how effective a charity is at advancing its stated mission.

Bill also looks at the effects his dollars have. He admitted that his attempt to fund smaller schools plain didn't work out.

Giving money is easy, but I guess he (and Zuckerberg) wants to give money where it makes a difference.

In this case it looks like it didn't make a difference -> failed gift.

I imagine it was because making good choices and judgments about who deserves it is difficult. Making sure the money is spent in a manner that matters, as opposed to given away as salaries to a bunch of consulting groups, is very hard.

I may be wrong, but it's quite obvious to me. You wouldn't want to donate to an organization where the money doesn't make a traceable and measurable impact. Luckily I see more and more charities prioritize transparency.

I'm sure he means that doing it responsibly (having the money actually be spent correctly and not wasted or lining other peoples pockets) actually takes a lot of time and effort.

I bet you he was speaking in the context of it being difficult to donate money effectively to see the most positive outcome.