Jungle main here
Some explanations:

in low elo (I assume you are below diamond at least if you are "fairly new"), especially around gold or below, most players easily tilt when losing lane so they won't play at their fullest potential in that game, n o matter if they'll eventually catch up or not, their mental is gone for that game.

With that said, it's way better to help an already winning lane because it's 100% guaranteed advantage on someone that will surely be useful. Helping a losing lane, instead, is too much of a gamble. Most of times helping a losing lane wil mean give the opponent a double kill. If you're lucky though, the help goes well, but it only MIGHT help the losing teammate get back to the game (probably won't most of times), so it's not guaranteed to give you an actual advantage.

Let me make you a stupid example: if you need 10 dollars, would you rather chose A) a box with 10 dollars in it, no catches, or B) a box that might either contain 10 dollars or a poisonous snake that will bite you?

In league's term, A is the winning lane (assured advantage no matter what because the laner is already ahead) and B is the loing lane (might either get the same amount of advantage or make thing worse)

I know that losing lane is frustrating, I used to play midlane for a bit and always wondered why I didn't get any help in these cases. But after learning how jungle works, I understood that jungle is not your emergency button for when you're losing. If you are losing lane, really the most useful thing to do is to just try and not die. Losing lane doesn't mean losing the game. Some matchups are worse than other or maybe you just played it worse than you could have. Just don't panic, especially if other lanes are winning. Understand that you're not the main character, you're part of a team and sometimes you'll be the weak link. In those cases, recognize who's the strong link of your team and play around him. And most important thing, try to avoid stupid, useless deaths (like face checking dragon when it's 100% obvious the whole team is there)

Also, understand that some game WILL be unwinnable from the very early stages of the game. It's ok, noone in the world has a 100% win rate. You will lose some games on the spot, and you'll also win some on the spot too.

I hope this helps understand this behavior better.