In the movie, Inception, Why would Saito and Cobb not go back to reality?

I understand the end where it doesn't show weather or not the top stops spinning. My question is why would they stay in Limbo? Is it because they thought Limbo was better than reality. Or was it because they couldn't get out of limbo to return to reality by killing themselves? Or do people doubt they went back to reality just because it didn't show Saito and Cobb killing themselves. So basically why would people doubt that Cobb and Saito don't get back to reality?