What excuse will the Wikimedia Foundation use when their fund drive comes up short?

They're not going to allow themselves to be limited to just one. I would expect most, if not all of the following to be used.

"We meant for that to happen"

"For some reason, there are people who persist in thinking information should be 'free'"

"I guess 2,000 photos of white guys' penises just aren't enough"

"Wikileaks: Excuse-wise, it's our version of 9/11"

"The 'Personal Appeal from Jimbo' banner ads were too small"

"Things should improve once the income tax is abolished and all men are allowed to pursue lives of uncompromising individualism"

"It's all the trolls' fault! Curse you, trolls!"

"Our unpaid amateur volunteer editors aren't working hard enough, the fat lazy basement-dwelling b*stards"

"We need more women - and this time, not just the ones who are actually transvestites"

"Somehow people have discovered that if you scroll down, you can sometimes see more Google results than just the first two"

No excuse is needed, because the holy mission will be successful. Praise be unto Jimbo!