Distillation is actually a fairly difficult and quite dangerous process. If your apparatus isn't within relatively tight tolerances, you'll get a fuckton of impurities in the end product that will seriously fuck you up. MOst backwoods moonshiners are from a long line of moonshiners, or know someone who is, and thus know how to do it right because "I learned it from my pappy, who learned it from his pappy, who learned it from his...", but when Prohibition came down, the resulting alcohol drought meant that producing liquor -ANY liquor- could make you a fortune, so a ton of city folks started manufacturing booze with only the barest idea of what they were doing (much the same happened with methamphetamines in the 90s and today, but failure to make meth correctly results in a rather large explosion instead of a visually identical but poisonous product, so the problem is much more confined) and got it wrong. That's cause 1.
Cause 2 was the fact that ethanol was -and is- an incredibly important industrial chemical, used as a solvent, a fuel, and probably a dozen other uses. To stop people from cracking open a bottle of cleaner and drinking it, a small percentage of poisonous wood alcohol was introduced into the mix long before Prohibition (factory owners didn't want their workers getting drunk on the floor with company property). At the original levels, it was just enough to make you pretty sick unless you drank a massive amount of it, but spikes in the theft and back-of-the-truck sales (often winked at by the owners of the stuff) of industrial alcohol caused certain insanely overzealous regulators (many of whom had been pushing for Prohibition in the first place, got their jobs because of their anti-booze stance, and stood to lose everything if Prohibition failed) decided to mandate a much higher proportion. That would have been bad enough, but the folks that were drinking the stuff would mix it into punches and such to disguise the taste (industrial alcohol is very harsh stuff), and that kept people from noticing that this stuff tasted extra bad, and it didn't get you as drunk, so you drank more of it. Hundreds, if not thousands, died or were permanently crippled as a result.