My best guess from this limited info is probably because your aging core 2 duo from 2008 is approaching 6 years old and is probably not up to it. League of legends is competing in a world of i5' i7's where Layer 3 cache is king , im also going to make a guess that its not just LoL you are running , on the chip , its windows , all your background applications , some form of voice coms IE skype , you probably have a couple of browser tabs open eating Ram like a greedy child.
Maxed and high settings are really only for machines built within 2-3 years of the application and while your pc maybe wasnt built in 2008 , that Cpu was .
Sorrry buddy