It's not because they're "immature" or anything like that, and it's not a diss on anyone who still likes them, it's just that I tried to watch a new video of theirs and it just didn't hit me like it used to.
It's similar to what happened with me and the Nostalgia Critic (Tho admittedly with significant differences): I was a pretty big fan early on and for several years, but over time just fell off, started watching them more sporadically, before kinda just not keeping up with the new stuff anymore.
I've found that it's natural for me to go through phases where I'm interested in a particular series/piece of media. I'm willing to admit I used to watch The Nostalgia Critic regularly, and then gradually stopped, partly because of Doug Walker and Channel Awesome's well-documented awfulness, and partly because, like you said, I outgrew it and had moved on to other things. I found I wasn't as interested in his videos anymore.
It will probably happen with the things I currently spend my time doing on the internet as well, e.g. watching Red Letter Media, using Hexbear. I went through a year during which I was a regular Chapo listener, and even that I don't listen to as often as I used to. I hope that I do grow out of the things I'm currently into and find new interests -- otherwise I'll become a boomer who complains about how kids these days don't like Led Zeppelin.