There are various reasons for this:
Your cryptocurrency is usually made available in your wallet as soon as we receive your payment. So, your transaction will be “pending” until we receive the money from your card service provider. Also, depending on your location or card service provider, your payment might take longer than usual.
The transaction speed depends on the speed of the cryptocurrency network because it has to be confirmed by the digital currency network. So, your crypto might be delayed if the network nodes lose sync. This doesn’t last long, though, and the transaction eventually goes through.
If the digital currency network is experiencing an extremely high volume of transactions globally, and there are more transactions than available space in each node block, your transaction might be pending for a while. This is a rare occurrence, though.