Don't know how these two images (animations?) look in their natural environment, but I can think of two possible explanation why the latter image is more popular:
1. Just by looking, the boss sprite is clearly more visually interesting. The big sprite clearly grabs my attention, and I can see some cool details about it.
The way the boss fight picture is posted here, I can't tell any details at all, and there's not much to focus my interest on, except maybe for these lighted crates (?) and this purple cloud with undefined figure in the middle.
2. Maybe the latter picture is more lucky? Like, it was posted at the time when more people were around and could see it, and a somewhat prominent initial traffic could cause a snowball effect, when popular thing is favoured by algorithms and becomes even more popular?
Maybe it's a bit of both. Again - I don't know if these were just the pictures posted as they are, or they actually appeared larger or maybe one of these was more animated.
However, the way these pictures are posted, I definitely consider the latter to be cooler looking just because of the amount of detail. I'm pretty sure if I saw this boss sprite - about as large and detailed as in the second picture - but as a part of finished gameplay screenshot, I'd find the overall picture at least as cool, and likely even cooler.
Note: All that comes from a person who is completely insensitive to retro nostalgia and other appeals like that. Thus, an additional "indie feel" of any kind doesn't really hold any added value to me.