Well, I'm not a jungler myself, but I'm subscribed to several higher-elo junglers who have spoken in regards to this topic.
The reason many junglers do not gank losing lanes is this; if a lane is losing due to a lack of jungler influence (especially from both sides, not just their side) then that means the player(s) your teammates are playing against are simply better. Meaning that even if you were to get them back on their feet, odds are they'll just throw any lead you give them. Not always the case, but it tends to work out that way.
If a jungler ganks a lane that is already winning or going even, then they have the option to extend that lead even further, opening up more chances at securing objectives and creating pressure on the map. If done successfully, the lanes that succeeded can help carry the losing lane(s) to some form of relevance through global gold or buffs via Dragon.
Now, a jungler MIGHT gank a losing lane if the following applies;
- The lane was influenced by the enemy jungler, so it may have just been an accident that can be recovered.
- The lane has a powerful late-game/hyper-carry champion who, once recovered and played correctly, can carry the team later into the game.
- Doing so opens up the opportunity to take certain objectives (towers, dragon, rift herald, etc.)