Actually Champs =/= skins.
See there are multiple different groups working on champions at different times. Each champion has a different designer/set of designers and multiple champions are being worked on concurrently. Sometimes a champion goes through the process really quick, other times champions like Azir or Ao Shin go through the process in long drawn out periods. Riot releases these champions when they are done the same way they release skins. If a champion like Rek'Sai takes 6 months to go from start to finish and a champion like Kalista takes 3, they come out with Kalista and realize a few weeks later they already have their next champion out.
It's not that they didn't take long to think of Rek'Sai it's that it was a totally different group of people working on Rek than working on Kalista so development was happening concurrently.
With champions there is no deadline, when a new concept is made and finished that's it. With skins it's a lot different. They need skins for Lunar Revel, they need a skin or two for Valentines Day, they need a Championship skin, they need Harrowing SKins, they need Snowdown skins, etc. Other skins like Arclight Vel'koz are fleshing out skin lines. Varus had a really cool skin concept and it felt like a concept that Vel'koz could expand on.
Instead of releasing another ahri skin
When you're making a big skin, I'm not talking about a theme skin like Arclight, you don't want to pick a new champion or a champion that nobody uses. A Championship skin represents the whole season. The team skins represent their picks that changed the competition, for a whole season. The Riot skins are easily recognizable champions that had play/traction at the time their Riot skin was released. You don't go 'Oh I released Rek'Sai last month and I need a champion to represent this whole season! I better pick this new champion whose identity nobody understands and who might not even be played all season', no they went with a recognizable face (Ahri) and a champion I'm sure they were playing around with changing (they were probably thinking about DFG changes at that time). They went with a face that was LCS familiar over a champion like Azir who didn't have a lot of play yet.
Thresh got a championship skin early after his release because let's face it he became a staple support quite quickly and he became a recognizable part of bottom lane, arguable a staple of the game.
If you wanted to make a second skin for Rek'Sai or Azir you'd pick an actual theme to explore. Undead Azir, Underwater Rek'Sai. You don't slap that year's banding skins on their name.