Why can a transaction take time to become fully confirmed?

"A transaction may be accepted by merchants before final confirmation through payment processors that temporarily bear the risk of non-confirmation, but the underlying blockchain process requires miners to include it in a valid block. The bitcoin protocol also delays a miner’s ability to spend newly awarded bitcoins until ninety-nine additional blocks have been added, giving the network time to develop strong consensus about the original block. If two blocks are found at nearly the same time, the network chooses one chain to continue, and the other block becomes orphaned with its reward invalidated."