Riot's resources and dominion
Gonna dissent from popular opinion here, so feel free to downvote but:
It's not that Riot doesn't have the capacity to support dominion. We all know that they do, as they did it years ago, before they had the revenue they have today. There is the idea, however, of opportunity cost.
Dominion requires a few things to make it more desirable: a graphical update, balance, inclusion in lore, and hype (the way that urf and other rotated modes are hyped up). This means it needs attention from the map team, the balance team, the lore team, and the marketing team. Yes, Riot has all of these teams, but deploying them to dominion would make it impossible to meet the standard of maintenance and improvement on the Rift.
"But Gingerfap, they have so much money, you KNOW they could just hire a few more people"
Well, it's not that easy. Suppose they doubled the map team so that they could support both maps. Dominion gets updated, but ongoing upkeep isn't as important or intensive as summoner's rift and dominion serves a minority of players. Riot is now overstaffed and needs to let people go. Temp positions are poor treatments of people who could otherwise get a long term job.
Couple other issues I see with it, but here's a little food for thought.