T20 as a concept was to make the cricket fast paced,as short as possible and make the game appealing to all people (something like how baseball is in US). The idea was to make each innings last only for around 90 minutes so that it attracts more people to watch the game. One of the ways to reduce the time taken by batsmen to occupy the crease was to make them sit near the boundary.
The prime reason why Batsmen sit close to the boundary in the T20 matches is to reduce the time taken by the next batsmen to occupy the crease. Also it gives flexibility to send out any batsmen according to the team's current situation.