Well, according to the new book "Sesame Street 40 Years of Life on the Street" When talking about David Rudman and the character the book says;
"After noticing a lot of R words, like porridge, in a script, he got the idea to lisp the letter. The character quirk made everyone laugh, so writers developed Baby Bear scripts."
And personally, I always assumed that the character had a lisp because he was supposed to be the "BABY" bear from the story, therefore he talks like a baby would. Even though technically he is not a baby anymore, (as mentioned when Curly Bear arrived) its still a fun character trait.