Well, you've already heard Pheonixent's and MoviePropMaster's explanations for why 9mm guns are much more common in movies than .40 or .357 SIG.
The P229 in 9mm didn't become available until quite a few years after the original .40 S&W and .357 versions. That's why the P228 remained in production until 2000. Since armorers used to prefer buying 9mm guns for blank conversions, they would have been buying far more P228s than P229s because they simply couldn't get the P229 in 9mm. Years later, even after the P228 isn't in production anymore, they've got so many in inventory that they still get used in shows like "24".
Also, the fact that P228s are no longer produced and harder to come by is irrelevent. Remember that the guns you see in movies/TV shows are usually
rented, not bought and converted to blanks specifically for that particular production. The P228s you're seeing in this season of "24" right now, for instance, have probably been used in dozens of other movies and TV shows in the past 10-15 years. As long as the armorers still have them in inventory, it saves them the trouble of having to go out and buy new 9mm P229s when they've already got lots of P228s.
I'm of the opinion that the P228 is a better gun than the P229, anyway.
