As stated on the main page, one of the main requirements of this trope is that it doesn't count if the story takes place in a World of Ham, or if the hero is also a Large Ham, unless the villain still manages to out-ham everyone else in that situation. Therefore, the very existence of this trope depends on the villain being different, or a foil, to the hero. For that reason, when you look at some common hero and villain attributes as we're about to do now, it makes so much sense that villains would be the biggest hams.
Let's start with the hero. Most plots in fiction are driven by the protagonist wanting something, whether it's self-fulfillment, saving someone they love, etc. They will take the thing they want very seriously, otherwise, they would come off as too flippant for the audience to engage with them. (Unless it's a comedy, in which case it's a little more acceptable for the protagonist to be non-serious. This usually leads to an Unsympathetic Comedy Protagonist.) Because they take their goals so seriously, most heroes are thoughtful and caring, which can lead to them being gentle Nice Guys, or maybe dark and brooding heroes (if not anti-heroes), etc. A serious hero has restraints on what they will or won't do, otherwise they would be arguably much less sympathetic. It stands to reason that someone who is restrained enough to not lash out at anyone who wrongs them in any way would also be restrained in attitude, and so they would act a little more quiet and civilized, since that's how people are generally expected to act in the real world.
Now, the villain is typically the antithesis of that. They don't care about the things the hero does. Their purpose is to subvert and destroy everything the hero holds dear. The thoughtful and caring elements of the hero are usually completely absent, and that often translates to the villain being a lot more boisterous, unfettered, and sometimes informal. This characterization may not be sympathetic, but there is something exciting about how unrestrained these villains are, and that's likely the reason why some of them are so popular with fans. There's an element of escapism to it. Some people like to watch a character who does not abide to the restraints that everyone else does.
