The other suit might still have a few deadly snakes without being full of them, and we don't know how many harmless snakes (or anything else) are there.
All of the reasons already mentioned, but also, your brain is hardwired to pay more attention to a face, and to a face expressing emotion even more in particular.
The first two panels establish a pattern that your brain subconsciously keys in on. At a glance even without registering the specific images and text, your brain instantly knows that panel 3 doesn’t fit in the pattern.