An addendum to your post:
Malphite bans are not as common due to the larger defensive mastery tree that rewards higher scaling top laners (where malphite usually lives), and so high resistance / health bruisers get better buffs through Perseverence + itemization of resistances (which are more gold effective than health at this point).
Ammumu bans are not as large due to conservation stacks, ammumu isn't a "carry" jungler, and his scalings don't hugely provide a benefit to having the extra gold vs having a 2nd carry on the team (the super 4 at this point: vi, leesin, elise, and kha'zix).
Also to note on both the blitz + ammumu bans. Blitz Q + Ammumu's Q + R are about the only massive things in their kit (don't get me wrong, they're amazing in the right hands), but after that, they have little sustained damage or use in the team, versus Thresh, Leona, Annie, Zyra, who all have much better sustained CC + dmg in their kit.
Hope that explains a little!