With Cena, it's cause no one has really gotten to his level. CM Punk got pretty close, but Cena is just far more marketable. Daniel Bryan was widely loved cause he was a underdog and there was just something about him that resonated with people, but at the end of the day his mic skills kinda lacked and was injury prone and retired.
I don't think Cena should always be the champion, but at the same time, keeping him around in some form of spotlight is good business. Like, his charisma and mic skills are good for pushing Heels to bring out more heat when they make comebacks, too. WWE is more about how you appear than just mere skill, which is why Cesaro for a while didn't get much of any push. He had like all the skills of a champion, but was terrible at the mic.
Roman Reigns is just... (not sure if he's improved since i watched) but his mic skills are just awful, lol. I'm not really sure if he can ever be a real contender for the company.
Ryback has his place as a simple macho dude, but that's about it.