1 Rowan Atkinson got older
Rowan Atkinson, the actor playing Mr Bean, said that he got older and that he shouldn't be playing something childish at that age.
2 Mr Bean should be young
Atkinson said that Mr bean must be young and that he doesn't like the idea of Mr bean getting old. He said he prefers to leave Mr bean at that ageless and timeless zone.
3 Acting childish makes one sad
Atkinson said that when a person reaches his 50s, acting childish can make him sad, so a person should be careful.
4 Change in comedy tastes
Atkinson said that these days most comedy shows aren't family shows but they are more directed to grown ups. The fact that comedy tastes changed over the years could have been a part of the decision.
5 Atkinson doesn't want to get stuck
Atkinson hinted that being stuck in one character could prevent him from taking on different roles. This could have been one reason behind the decision.
6 Atkinson wants more serious roles
Some people suggest that Atkinson wants more serious roles and that's why he stopped playing Mr Bean.
7 Doesn't want to be known as Mr Bean
According to a thread in answers.com, Atkinson didn't like the fact that people knew him as Mr Bean and he wanted to be known as himself. See interview with Atkinson.