In theory, the pro-bowl represents the players that have had the best season from week one through approx. week 16 (when the voting is finished). To include the playoffs, you would either need to look back to the last season, or ahead to what you
expect to happen latter in the current season.
Most people realize, however, that the pro-bowl is often a year behind, and often rewards players for what they did the last season (including playoffs), or even beyond, rather then what they have done to deserve the trip this year. Many a good player has been slighted because another player with a great career, but lousy season, was given his spot.
Just the nature of the beast though