'
Meg' has a huge hype train behind it - it's been famously held up for years. Plus, there really hasn't been a serious attempt at a film dealing with megalodon. Contrast that against another '
Predator' film and casual audiences would at least be split, if not have a preference for the novelty of the giant shark film.
Plus, there's the 'date film' angle... Guys have a better chance convincing someone to go to a generic monster flick than one which famously revolves around skinning and ripping skulls out of people.

'
Deadpool' was more subversive and had the comedy angle going for it - it had wider appeal. You have to play release dates carefully when it comes to something with the type of subject material '
Predator' films do.