Nexus siege failed to deliver on the RGM fantasy.
RGM should be engaging, weird, and fun. Nexus siege just makes you feel like everyone is playing heimerdinger.
Hexakill is chaotic because an extra champions allows for better comps and strategies.
Urf is urf. It speaks for itself.
Nemesis draft requires you to think outside the box to get stuff done.
All for one is crazy fun because it allows you to abuse champion synergies (5
. oh god no).
Nexus siege is just kinda, there. No one finds it fun because it takes one of the grindiest parts of league (the sieging), and bases a whole gamemode around it.