Because they are dead and are meant to stay that way, while heads frozen in cryonics are meant to be revived. Severed heads also have faces so they are more human-like than vat-brains so they can evoke a greater emotional reaction.
Yeah, I think it's mainly the heads/brains distinction. Even though a person is much more their brain than their face, we don't intuitively think of brains as people the same way we think of bodies/heads/faces as people.
Note that the author in question didn't discuss cryonics at all. The author might consider them to be just as cool. I suspect that in general there will be a high correlation between the two.
One thing that might contribute is that the Cushing/Whitney Medical Library was designed by skilled architects, whereas the photos I've seen of Alcor's facility look more like the inside of a ticky-tacky office building.
I find the collection of vat-brains creepy.
So why is it described as cool? Because I'm not the person who so described it. People can be different.
Why is this collection of vat-brains described as "cool" when cryonics - frozen, severed heads - is described as "creepy"?