There are quite a few reasons.  One of the reasons are that the Region was a Gentile predominant area (note the pigs as jews are not allowed to eat or touch them) and they were probably afraid that he might do the same to their other pigs. They were more worried about their livelyhood than about their salvation. (just like most people reject Christ today)

They also probably feared the unknown. Christ had power over demons and this scared the gentiles.

Not has changed much in Man's reaction to Christ.