I love this thread idea Zobo and have to give props to you for thinking about it.
I guess that you have to look at exactly what kind of monster the first was. It wasn't a combat tank like Adam or Glory. It wasn't diabolically suave and intelligent like Angelus. Nor was it visually scary like The Master and it certainly couldn't compete with Dark Willow as a magical entity.
What it was really, was insidious. It was the nasty little voice inside everybody, the one that whispers in the dark. I think it was actually too cocky and liked the sound of its own voice too much. It thought it could outwit its enemies, play games of destruction and insanity with them. Most times, it chose to appear to people as those they held weaknesses for, like loved ones. Spike saw Buffy, Andrew saw Warren, etc. When it did manifest in other guises it was often as the person themselves, Buffy and Buffy, Spike and Spike. This was to play on their self doubt, weakening their decisions and determination or resistance.
Whenever it did show up as other characters, like Chloe or Eve, then that was to work upon potentials mostly, of which we didn't get to learn enough about to know who was their weaknesses, so it didn't really matter. As a side bit of mind games, doing so did then throw into the mix the terrible thought "it can be anyone" so thus sowing seeds of distrust amongst the whole group rather than individuals.
When it showed up to Willow it was written to be Tara, but Amber wasn't available so they had to hurriedly rewrite it and used Cassie, purely down to availability.
All that said, there is no denying Zobo's logic, or its staggering validity, so yeah, "Why didn't it show p as Buffy?" 🙂