The time limitation exists to ensure that as many fans/customers as possible are given the opportunity to buy tickets.
More specifically, when you choose your seats, they are reserved and the tickets are added to your basket so that no one else can choose them. There is a limit on the duration you can keep your seats reserved, in order to give other fans the chance to buy those tickets if you decide not to.
After all, it wouldn't be fair if, after some time, other fans/customers who really want to buy them and are ready not to because there is no tickets availability.