That's mostly because they DO NOT have a mature business model. Reddit struggles because their main source of revenue, ads, performs poorly on the generally tech savvy audience that makes up the core of their platform. They have been trying to figure out for literally the entire life of the company how to turn a top 10 visited website into actual consistent and repeatable revenue. You'd think it be pretty simple but look how much push back they've gotten on even relatively benign monetization strategies.