[solidsnack9000]

Suica is a stored value card, so you don't have to do an online balance check to validate a transaction.

There is also probably a practical element to it, in that Suica and other IC cards are managed by entities that are closely linked to the various train systems in Japan. The terminals are known to them, the cards were all issued by them; they can probably preload a lot of stuff at all the train stations and keep the data regarding the cards synced directly among the stations. (It's at least feasible that by the time you get to the next station, 90% of the time, the information about your card is already there.) This is in contrast to what would happen if you used Mastercard or Visa to pay for train fares, where all the data would first have to go to northern Virginia...