Every skill has both a cost to maintain and a cost to learn. Take the case of learning a language: You need to spend some time every day rehearsing vocabulary and grammar you already learned so you don't forget (maintain), then you can also study new words and grammar points (learn).
The maintenance is a fixed cost - you need to spend this time just to not forget what you already know. As you get better, this cost goes down. Either because you can integrate the skill in your normal routine - read news in foreign languages - or because it has become so ingrained in your body that it doesn't need conscious thought - picking up a bike after many years of not riding one.
If you try to learn many things at once, just the cost to maintain them can easily eat up the daily time you have for practice. Instead, pick up one skill, focus on it intensely until you master it and need very little time to maintain it, then move to the next.
I noticed this when learning Japanese: I'd spend 10-15 minutes every day just reviewing old cards in Anki, not even studying anything new. If I half an hour every day and I also want to practice piano and meditate, I'd never improve! Instead, I decided to focus solely on Japanese, to reach a level where I can comfortably read news and books to passively maintain my level.