If you think about it, that's how matchmaking works.
You're doing well with a good W:L ratio, so the game starts to pair you with players that are average or below average expecting you to carry, or to see how well you do. If you keep winning consistently, you'll skyrocket up in rank.
However that's not reality, because in reality we can't always carry a team of four significantly lower skilled players -- or always win against stronger opponents as a group 5v5 -- so you lose games against stronger opponents or with teammates that are significantly less skilled as a whole.
This is done in part to keep players from limitlessly skyrocketing their W:L (and therefore MMR), as by intended design the LP gains/losses are tied with your MMR.
Think of it like being on a see-saw: you have five players on each side; let's now say everyone's on fresh accounts with the default MMR and are essentially 'blank slates' to the matchmaking system. That means the see-saw is at perfect balance. Over time, these players' MMR change, therefore they change the balance, skewing the see-saw heavily to one side. The point of the matchmaking is to attempt to balance the see-saw back to 50%/50% middle ground, but at the same time it needs to test the limits of individual players' skill level, and it does so through pairing high skilled (high W:L) players with less skilled ones, in an attempt to gauge their abilities in order to better set their MMR.
It's all checks and balances; if you had no "skill cap" with the MMR/matchmaking system, you would just continue spiraling out of control in one direction with either a win or losing streak, and that's no good for the system and the rank system.
So from a design standpoint it makes perfect sense; from a realistic view however, it can become flawed in the sense that the player base itself is more polarized than previous seasons between skilled and knowledgeable players and new / unskilled / troll players. I also feel smurf accounts skew the system a bit in a negative way as well. I think in order to give the matchmaking system better player matching prediction abilities, Riot should invest some effort into adding more data points for determining an individual's skill level, not just with W:L ratios and/or whatever other data points they use for reference. Maybe adding something _in addition _ to the MMR number as a skill reference, could help balance matchmaking more precisely.