There are a few things that contribute to this bitcoin has a block size of 1MB and they only add new blocks every 10min. There are only so many transactions you can fit in that block size. The other thing is as the block reward gets half'ed miners will ask for more fee's as well as miners will mine blocks with higher fee's first. So as a way to combat some of the congestions people increase the amount of fee they are willing to pay to get included into a block faster.