People who thought neither choice was suitable had the option to write in "None of the Above" or any desired alternative.

Even if one hates both choices, it's usually possible to decide which is the lesser of two evils. And if one doesn't vote for the lesser of two evils, one risks the greater evil winning.

I suspect that if people had thought the vote was going to be close enough that voting NOTA would make a difference, more people would have voted for it.