Dominion died because people simply got bored of it. TT died because Riot updated the map and added new gimicks like the altars.
Snowdown Showdown was a failure because Riot compared the playtime of One for All (30-40 min games) to it (1-5 min games)
Nobody got bored of Dominion. Dominion isn't boring. Riot didn't support it at all, blatant imbalances were ignored, new players were clueless about what to do and personal score led them to play like retards (still does). Each new champ was more and more broken on Dominion with very few exceptions. It took them years to finally remove crippling map-wide "buffs" (that were totally unwarranted to begin with), but by then it didn't even matter anymore. It became a joke because Riot didn't care about addressing its issues while continually introducing new ones.
TT is technically not dead yet, because people still play it for ranked. But most players who refuse to play it do so because of champion imbalances. The "gimmicks" don't matter that much, and you'll see when people don't quit playing LoL after SR gets "gimmicks".
Snowdown was a failure because 1v1 in this game is and always will be fucking dumb.