I believe the main reason was because if there is a bank, pkers would flood the area around it and kill everything in sight. (just look at level13 on Classic.
Also, banks were brought up many a time... We were going to make a bank where people could have safes there, and thieves could rob and all that, but it would just bring too many complaints.
Personal house safes are better and you could rent rooms all over the island to have money put away everywhere
