"Vanilla" is still the most popular flavor ice-cream...Toyota Camry is still one of the best selling cars. Neither are the most "exciting" but they are tried and true.

Combine that with the old saying, "People may not know what they like, but they like what they know."

Buble presents familiar material in a very palatable format for what I call, the "common ear".

As Mark said above, "He picks really good songs." They are "old reliables", and he (or his manager) knows his target audience very well and gives them what they know.

Just my 2 cents.

Ian