KI already said what I was going to say: The sheer simplicity of rage comics is what makes it so appealing, particularly to simple minds. But I'll go into more detail
Rage comics are such an easy concept to grasp. They consist of basic characters with only one single trait where it is easy to depict that character doing what it does, even if you are by no means a comic genius.
If you don't know how to make a comic, you can learn how just by making Rage comics. They are as basic as comics get. Essentially, for many, they are the first stepping stone to generating internet humor
Rage comics are further complimented by easily replicated pre-set formats that anyone can copy. No skill required
You have 4 panels, MS Paint and a featured a rage character that you can easily copy from somewhere. If its rage guy, you depict him raging at something that makes you rage. If it's Gusta, you depict him liking something you like. If it's Forever Alone, you depict him feeling rejected by something that makes you feel rejected. Simple, simple, simple.
Anyone can do that whether you are young or old, smart or stupid. The rage comic openness, flexibility and versatility to idea's all equates to a meme that easily spreads like wildfire
I'm glad that KI also mentioned Advice Animals because Advice Dog is popular for that exact same reason. With Advice Animals you just take an image that is either a thing itself (Teacher, Liberal, Christian, Soccer Mom, Politician) or represents the thing you want to depict (Paranoid parrot, Awkward Pengiun, Foul Frog, Courage Wolf) and you point out it's hypocrisy and contradictions or just character traits.
You take your Advice Animal, put a statement at the top about what it does then a statement on the bottom about how it contradicts the first statement….that's it, done. Again, a beautifully simple format that is easily to replicate and morph no matter who you are.
So long story short, it's all about simplicity that also does not restrict creativity.