A Bitcoin block can only handle 1mb of transactions, currently all found blocks are fulled, they have over 999.11 KB, so you have to wait until all previous transactions are confirmed until they get to yours. This is of course unless you send a very high fee to get your transactions in front of all of the rest.
You can check the blocksizes here: https://blockchain.info
EDIT: A bitcoin block is found every 10 minutes, so it can take a very long time until your transaction is confirmed.
it is time to have segwit already... zzz...
This time waiting is sometimes excruciating. I just sent some Bitcoin with Coinbase and it took nearly an hour to get the transaction through (that was with a fee of $0.56 too). I believe this is going to get worse before it gets better.
coinbase is the worst, its taken me hours to send btc to another wallet or market .... i think thats a mixture of coinbase and time wait for btc too tho... but in one case i ended up sending btc from my mycellium wallet cuz coinbase took so long in making confirmations
I am with them mainly for the Shift card. Cheaper access to my BTC means a lot. I don't usually send money from that account.