It's because their league system is convoluted and inefficient. Your MMR is what really matters but isn't tied directly to your tier or division. Instead they have this meaningless LP system that determines whether you climb and instead of being a static gain/loss with a modifier for win % they base it on a comparison to the average MMR of your division, which again isn't actually tied.
So when you go on a streak your MMR actually ends up low for your division instead of high like it was before the streak and you end up with this wierd clamping that shouldn't occur outside D1 like riot stated. The problem is their system is setup really stupidly.
Without going into too much detail here is what they should have done:
- kept MMR visible and tied tier/division to specific MMR ratings
- made MMR gains/losses a static number with a modifier for win rate (%)
- ????
- profit
It's a much cleaner system, you could still do promo series and there would be multiple approaches. You could have the promo series not add or remove MMR, they could add or remove a specific amount or they could be variable depending on games won/lost. You could also do demotion series (play to remain in your division). Oh and since it would all be based on MMR the matchmaking would be easier and you'd always get other players in their series too, instead of getting paired with random people. So the promos would be more competitive.
