You dont... Now in all seriousness let's play channels are REALLY hard to grow due to the only real thing that makes you different is your personality. I recommend working on commentary, build that skill up as much as you can and it can take you far. Then if you feel like you need to, cut out the boring parts in gameplays. For example: If I was doing a skyrim lets play and I had to run from one side of the map to the other without fast travel and nothing interesting happened that'd be like 5 - 10 minutes of boring gameplay. Also, you need to make thumbnails that catch peoples eye. Maybe a colorful text or something. Right now you have a simple formula which works sometimes but I recommend at least adding a stroke to the text in your thumbnails.
On a side note: When/If you do commentary one of my biggest tips is to be yourself. If you have to you might wanna exaggerate your personality. Don't be someone else, dont pretend to like something and dont be monotone. When you start commentating as well, your gonna find your own voice annoying until you get used to it. I did and I'm sure most people did.