Overall factors for deciding who gets a new skin are:
-How many skins does the champ already have?(the more skins he/she has,the less he/she gets)
-How has it been since the champ got a skin?(if the last skin was released rather recently,then the chance for another skin are rather low)
-Is a VU planned for the champ?If yes,when will it be released and how much will it change?(champs who will get a big VU will most likely not get any skins until the VU is finished,if they are lucky they might get a low-priced skin like Eve did with the safecracker skin)
-What kind of events are coming?(some champs might not get a skin for now,because they'll get a skin for an event,so they'll have to wait for that)
-What the skin team randomly thinks up is obviously also important(because sometimes the skin team might have a good idea for a skin and want that skin in the game regardless of the above factors)
-Popularity is also a factor,the more popular champs tend to get a bit more skins,but unpopular champs can get skins designed to make them more popular(battlecast urgot was basically meant for the people who didn't want urgot to be gross,but cool) and popularity can quickly change anyway,so I'd say it is a rather small factor.
So even if a champ has already many skins,that means that he/she has lower priority,but its just one of the many factors that decide if the champ is getting a skin or not.
Lucians last skin was released during june 2014,while garens last skin was released during febuary 2013,thats why Garen got a new skin.
