no special reason, but the concept works for now :) it's just a food fad. someone thought of it one day, set up shop, kids loved it, then a lot of other similar shops open. someday, interest will drop, most of these shops will close and another new food/drink idea will emerge. currently, pearl milk tea is somewhat being marketed as the anti-starbucks.

it's also pretty popular with teenagers around china, taiwan and some other east asian countries. not sure about the us, canada & europe as qataribound says.