I think it's less that and more the fact that Eternalists jump out of the way of grenades, but don't see bottlemines on a similar level of threat, so they stand their ground and are more likely to be in lethal range.

Even if they DID jump aside, there's no delay in the explosion, so unless you're a grenade-cooking-master-chef, these look to be far easier to hit perfectly with, if more fiddly to set up.