Why exactly League system doesn't use MMR instead of LP?

Couldn't type a lengthier explanation as I was in a game.

I've actually thought about this before and came to the conclusion that perhaps Riot doesn't have a choice. I'll try to explain my thought process below, perhaps some parts are not true:

• Players need to feel rewarded for playing the game. The most common way for players to feel gratification is by letting them unlock achievements, progress in ranks etc.

• Riot implemented the Bronze - Diamond ranking to achieve this. Checkpoints in each rank also make it less daunting for the player to try to climb (smaller, achievable goals).

• Now the big question, do we use MMR or LP to control the ranking? I used to think that they should just use MMR, but then I realized that perhaps it won't work. There's a very high possibility that people will disregard the new system altogether, and just use their MMR to compare, as the ranking is pretty much just attaching a name to a certain range of MMR. There is absolutely no reason for people to even refer to the ranking.

• Therefore, Riot needs to implement a new system that replace the MMR so that players know that they are advancing, but yet not know what is their true skill level, and hence the LP system. In this aspect, I would say that they are very successful, because I almost never see anyone compare their MMR, or LP, it's always their rank.

• The Season 5 placement fiasco also made me suspect one thing: Riot's system allow players to climb the ladder just playing more. This can explain why people lose more LP than they win. However, I'm not very sure on this so I won't say too much.

• Another reason for the LP gain/loss disparity could be that LP increases (and decreases) much faster than MMR, and I believe this is the normal explanation given to people, so I don't have to elaborate too much. If this is the reason for disparity, Riot can improve the situation by simply decreasing the speed of LP gain/loss to match MMR better. Then again, maybe it's already at an optimal ratio, if not there could be even more complaints.

Edit: Another reason I forgot to mention is that perhaps Riot wants to achieve a different distribution than the one that MMR provides. As in (I'm making up values here), if you use MMR, there would be 50% of people in the low range, 30% of people in the mid range, 15% of people in the high range and 5% of the people in the top range. But if you use LP (or ranking), there could be 20% of people in the low range, 50% of people in the mid range, 20% of people in the high range, 10% of people in the top range.