The number of transactions that can be included in a block is limited. A block is generated on average 10 minutes (a bit quicker due to increasing hash rate) so

The number of transactions that can be included in a block is limited. A block is generated on average 10 minutes (a bit quicker due to increasing hash rate) so the number of transactions in an hour is also limited. Naturally the transactions that pay most to the miners are getting selected by them and included in these blocks.