Possible explanations:
Elves just take longer to learn stuff unless forced to by their enviroment (i.e., adventuring). Left alone by outside pressure, their mental facilities go into hibernation mode, and they spend their days doing nothing - the ultimate couch potatoes, so to speak.
Elves overcomplicate stuff. They don't learn what leaves are poisonous, they learn what leaves were poisonous to what elf in the entire history of the ages, and the songs commemorating that. Adventuring elves learn the neccessities as quikly as other races and skip the rest, having not the time to spend on such luxuries.
They are elves. Don't ask why they are so slow to learn stuff, then advance as fast as the orc ranger from level 1 to 2, just accept that elven minds work in mysterious ways, randomly slowing down or speeding up.