Its because Riot stacks the game to prevent people from climbing.
You can figure out what win rate you need to climb and see that no matter what you do that account will not.
If Riot gave wins and losses the same LP you would climb to your true rank because you would start to lose. Your win rate would even out to 50/50
But Riot makes it so LP is not even. You have over 50% win rate but to climb you need like 68%. You will not get that in low ELO on an old account.
because the system will stack the games to get your to 50%. It will do this by give your team known afkers/trolls/feeders and the enemy team smurfs that it is trying to elo jump out of lower ranks because it thinks you are were you should be and will not let your improve.
New accounts the system does not know if you are a smurf or not. It does not want to lock you to a rank so you will get better Match making.
They want you to stay and play and buy Skins.
This is the same as Netflix use to do when you got the DVD's by mail.
New accounts would get new movies right away when they came out. If your account was old you would have to wait sometimes over a month to get a new movies sent to you.
They did this because if new accounts had to wait like everyone else they would drop the service.
Riot give new accounts a grace period before it hard locks it.
Only way to break a hard lock is to basically cheat. (Boost and or Script)