By design, you always get more lp the first 50 or so wins to help offset the fact that most people spawn lower than previous seasons.

Since youve been playing in your elo for months your mmr has equalized a lot around it since, presumably youve been mostly sitting at your current 50 % winrate the system intends to keep you here until you start climb again by just being a better player and winning more.

Think of it this way, to climb from s2 to g2 on 15 lps a win, you need about 33 consecutive wins, however in reality youll need less if you win them consecutively (since your mmr will improve a bit faster and youll get extra lp). Now, 33 wins can sound like a lot or a little, in terms of 500 ranked games its not really *that* many more wins than losses, its like 266 wins to 234 losses