It is a well known fact that Riot has been grappling with this dilemma for quite some time. To please the crowd or to sustain the lifespan of the game. Rito has tried Arurf, making changes to respawn timers, dmg, cd and other balances to try and fix this paradox but to know avail. Though I do not know the ins and outs of riot, their design philosophy nor claim to be an expert in social engineering; I put forth my own theory.
Though that it is no doubt that urf is fun, it is my belief that the game mode causes match quality to decrease in other game modes whilst urf is in rotation. Urf sells itself to be a chaotic fun game mode where all strategy flies out the window, it is well liked and popular in that sense but to those who play league for the team work they won't be sold on it. As a result of this the game mode mostly appeals to the casual/new demographic of the game. Don't get me wrong veterans probably enjoy it too but in essence the amount of people crossing over to play the game mode isn't uniformly inexperienced and veteran, rather it's skewed to one side.
This skew creates a discrepancy between the expected ratio of vets to noobs in the casual play oft making the normal game mode harder for noobs and unchallenging for vets (noobs begin to get paired with people above their skill). Mean while the large proportion of noobs/ casuals in urf means match quality in that game mode also decreases because experienced people will be more likely to match with noobs (but it's urf so nobody cares). Their for the majority of the people who leave are probably new/casuals whom prefer the normal game mode but frustrated and pushed out due to the increase in difficulty.
This is all conjecture, take with a grain of salt.