The algorithm of bitcoin is set to be limited so when all bitcoin has been mined, the miners will no longer be able to receive block rewards because there are no more coins to be generated. But why this?
There will always be block rewards for miners because block rewards consist of two parts - block subsidy (newly mined bitcoins) and transaction fees. The former part has been and will further be halved every 210,000 blocks by an algorithm until bitcoin reaches its cap, which is a bit less than 21,000,000 coins. The latter part is what users pay for space within scarce bitcoin blocks. These are a part of the total supply of bitcoin and consequently don't increase it. When all bitcoins are mined, transaction fees will be the only component of block reward.