I can only speak for myself but a lot of the times there isn't that many stuff you can add to a tutorial series before you're branching out to something more specific and tailored to cater an even smaller and smaller audience.

Take my turn based series for example (I guess it's a shameless plug too). Apart from the pathfinding which i intentionally separated as their own series, right after part 1 I face the choice to either go for the more X-Com style turn based game or the more action point/rpg oriented design. As mentioned in another comment, views really plummet to less than half for each consecutive part but it's something I personally don't mind because I just do videos for what I feel like it's interesting for me to do (but on some occasions I do requests too). As for my turn based series, I went for the first style simply because the second style can really be separated to smaller parts that not necessarily have to be for a turn based game, an inventory system for example, you can really make it modular and add it to any type of game if you wish.

It's something that I did before and it worked out great, like my AI series , although still ongoing, I made a usage examples video to show that, even if we made this for a top down stealth-type game (like the old commandos games) it could be used for a third person game, fps, top down action etc. with just a few tweaks for each case.