The amount of transactions that can be confirmed by the Bitcoin network are limited. In average, one block is mined every 10 minutes, confirming 1 MB of transactions. However, if there is more transactions than what can be confirmed by the network, the queue of transactions waiting to be confirmed starts to grow very fast. This can be observed by looking at the size of the mempool, which represents the size of the unconfirmed transactions a node has to keep in memory. In january 2017, the size of the mempool started to grow very fast, and people started to speculate why.
As bitcoin adoption is growing, and the debate about scaling the blockchain stalling, it becomes less and less practical to exchange bitcoin for cash in a peer to peer fashion. Indeed, if you want to sell bitcoins and fix a meeting with someone, you will have to stay until the transaction gets confirmed by the network, and this can take a long time.