In an ideal world in which hard work, dedication & loyalty always lead to someone getting their just rewards, then damn near every wrestler to have been a major star in the past 50 years would have been a World Champion with the company at some point. In reality, however, that's just not how things work.
For one thing, there's just not enough room. As much as everyone wants to be a main eventer and/or a dominant World Champion, it just can't happen to everyone. There's not enough time, nor enough spots. Kane is someone that, in my view, has been among the most underrated wrestlers in the WWE. I don't think that he gets enough credit for his in-ring ability or his promo work. You'll be hard pressed to find someone of Kane's general size & character over the course of wrestling history that's had his level of ability inside the ring and on the microphone. They're out there, but not very many at all.
Kane is someone that will be in the WWE Hall of Fame someday. It's not as though he hasn't had a great career. He's won lots of titles, been involved in lots of big feuds with huge stars, his longevity is the stuff that most other wrestlers hope of having as he's still highly relevant & popular. I'd have had no problem with Kane having a run with the title 10 or 12 years ago. To be perfectly honest, I wouldn't have had a problem if he'd had a solid run last year now that I think on it. However, I can see why Kane hasn't had those runs. In terms of drawing power, Kane isn't on the level of someone like Undertaker, Triple H, Randy Orton, Cena or CM Punk. In all honesty, he never has been and he's at a point in his career where he probably won't be around much longer. If I'm not mistaken, Kane turned 43 this past April so I'm not expecting him to stay in the game for longer than a few more years. His best days are probably behind him and I think even he'd probably acknowledge that. The role he's in now seems to be as a middle man in the Punk vs. Bryan feud in which he can be beaten & keep Punk looking strong, thereby giving Punk a win over Daniel Bryan without Bryan's stock weakening as well. It's a role that Kane has played as regular intervals over his career and it's a role he's good in. Kane puts on competitive, quality matches in which he helps his opponent look strong in victory while he also looks strong in defeat.
Based on ability & work ethic, Kane is one of a number of guys in WWE that "deserves" to be champion. Realistically, however, Kane shouldn't be WWE Champion. Punk's been a strong champion for WWE. Fans are constantly interested in his feuds and he's one of the most over stars on the roster. Even though Cena is still the face of WWE, that doesn't render Punk's reign as being weak or irrelevant by any stretch.