They've made their bones by being a uniquely fun and social way of a certain subset of the population to share their lives without parents dipping their head in and seeing what they're doing, ala Facebook. The UI is designed to generally support this; it is confusing and messy and understandable only by someone who is innately familiar with the app, or unfamiliar with the general conventions of UX.
But what happens when that user base isn't big enough and you have to expand? What happens when you need to figure out how to sell ads in an environment when you have next to no demographic information to target against? You'd have to fundamentally change the business: algorithmic timelines, a more sensible UI, more chipping away at the anonymity - all of which will turn off their core users in droves.