So it really is the influential or "victorious" champions which get the skin after all...
Season 1 - I wasn't around to know how effective Jarvan was and how.
Season 2 - Janna had a 55% win rate seemingly all year (at least at high Elo) and almost no one understood why. This changed (went to hell) the next season when they decided to give supports gold through items so they nerfed all true supports into oblivion and said defiantly that the role wouldn't be taken over by mages. The role was immediately taken over by mages until many more changes (nerfs to mages). To this day Rito pretend they were right... looking at you Morellolol.
Season 3 - Elise's kit had it all and she was a super popular jungler in both the LCS solo QQ until she was nerfed twice a day for a year. Now she's only ever successful in the LCS and you guys keep picking her in solo QQ even though you lose 60% of the time. You know it and the stats show it, stop it.
Season 4 - Morgana being popularized in pro play was what finally stopped the practical 100% pick/bans on Thresh/Braum/Annie in the LCS. Without Morgana around bottom lane picks would be a lot more stale not to mention annoying. So naturally they nerfed the living shit out of her in a way that ensured that she could never be picked mid lane again (base damage was annihilated and no scaling damage was added to compensate for mid). Good job, better nerf Irelia.
The point: By this list it really does seem like it's that season's "Victorious" champion which does get the Victorious skin.
There is a huge problem with this standard... that means a mid laner will never get a Victorious skin. Whenever a mid laner had become anywhere near that popular and/or successful in the past (relative to the other champions at the time) it was nerfed repeatedly either into near-zero pick rates awaiting a rework Soon™ (give me back my Diana damn it), or at the least nerfed enough that it wasn't pick/ban in competition if it previously was. And this has always been done well before the season ends.
Two supports and two junglers now. Laaaaame!
, back then, was a dominate pick at all stages of the game at all levels of the game. He could go Bruiser, Tank, Assassin, and still be usefull. That said, you have to remember that in S1, the Meta golem was becoming king, being Sunfires, FoN, Atma's, Warmogs, Mercs, and Wriggles being king. You couldn't even TRY to build that today, the only item even remotely similar to itself is mercs, Sunfire was ramapant back then and early in S1 could be stacked, RIP FoN, Atma's got destroyed with nerfs, warmongs used to have a stacking mechanic for even more HP, Wriggles used to be the ONLY "jungle" item, but was so good that even top and some ADC's picked it up, as it had lifesteal and free wards on the cheap, not to mention flat ad and some armor.