If you're going to make it a question of access, then it seems Riot needs to do a better job of making their supplementary lore content more accessible. A lot of these lore pieces are one-off promo events that are discarded and difficult to find after the event. How would newer players or people just getting interested in the lore even learn about it or realize it exist?
Compare this to the Overwatch website by Blizzard
https://playoverwatch.com/en-us/media/
They keep all their media on one page. This includes film shorts, comics... it's an easy to use Netflix-like binge material for people who are interested in the expanded Overwatch universe.
Or Team Fortress 2 by Valve
http://www.teamfortress.com/
This webpage has all of the movies, comics, and even a history tab conveniently located on the FRONT PAGE so people can easily access this lore.
Compare this to the League of Legends Website
http://na.leagueoflegends.com/
You'll see a bunch of esports updates, tabs for 'Game Info', 'News', 'Esports', 'Boards', 'College', 'Merchandise', and 'Support'. Even if you click on the 'Game Info' tab, all you'll see is a bunch of basic, beginner stuff on the (retconned) Summoner's Code, and a bunch of stuff about the Field of Justice... basically this is still pre-Retcon after TWO YEARS. So how can one even find all this expanded universe stuff like Burning Tides, Bloodline, etc... if they don't know they're looking for it?
tl:dr - Riot's lore is not that good because it's not as easily accessible as other franchises.