It's to differentiate between Smurfs, and good players that got lucky.
I've never liked it (not being a Smurf), but in hindsight it's pretty fair.
If I'm a good player that deserves to climb, but happen to go on a win-streak that's more realistic of a high-diamond to Challenger smurf... I shouldn't miraculously jump up 4+ divisions just because I hit a hotstreak. I still need to earn it. Now, if I can continue to win more than 50% (or even just 50%) against high diamond players when I'm in Plat 3, I'm going to see big LP gains with small losses and rank up pretty fast.
However, if I'm not a diamond-calibre player and just had a hot streak, I'll win maybe 40-45% and thanks to the larger gains than losses I'll maintain my rank for a while and even gain LP in aggregate (however I'll probably lose promo series). So I'm stuck in Plat 3 and can't get Plat 2 because I can't beat diamond players and sure that sucks - but if you really think about it I maybe didn't deserve to be Plat 3 in the first place. I went on a hot streak after hitting Plat 5 that brought me all the way to Plat 3 really quickly, and if I won that promo series against Diamond players I might have skipped to plat 1 (like a high diamond or better calibre player should) and since I was really only plat 3/4 quality I wasn't able to win that series. Then I eventually had some bad-luck streaks (to counter that earlier good luck streak) and dropped to plat 3 0LP, got demoted to plat 4, and eventually lost my way down to Plat 4 0LP (despite still winning 22-24LP per win and losing 17-18LP per loss).
So, in reality, you climbed faster than you deserved to due to luck. As a response, the system being accurate, results in you not climbing at all for a while until the MMR of your opponents drops enough that you can win around 50% again. If your opponents' ranks at that 50% point are still higher than your current rank, then you'll climb due to the >20 LP gains. If they're the same rank, you are where you belong, and if they're lower you'll drop.
i.e. in summary: I got from Gold 1 to Plat 3 100LP due to a period of pretty awesome luck (had a few 8W1L streaks in there) and probably never deserved to be Plat 3 100LP in the first place (and definitely not to climb that many divisions that fast), so the system catches me there until my luck balances out and figures out where I belonged.
Everyone always thinks they're great and deserve to rank upwards. In reality most people are accurate, and as many are overranked as there are underranked.