The simple answer is that there is a constraint on the total number of coins that will be generated.

In economy the fact that the supply of newly minted coins is either constraint by a constant rate at which they enter the market or, like in Bitcoin's case, even that the total number that will ever be in circulation be limited.

Due to how the amount of newly issued coins in the Bitcoin network is reduced over time the total number of coins ever issued will converge to 21million. Every 210'000 blocks (or 3.993 Years in expectation) the number of new bitcoins that are issued in the network is halved. The result is that we have a sum of a geometric series, with each step being multiplied by 1/2.

Hence in the first step 50 * 210'000 = 10'500'000 coins were issued, in the current step half of that will be issued: 25 * 210'000 = 5'250'000 coins and so on. The sum of a geometric series that halves at each step is exactly twice the first step, hence 2* 50 * 210'000 =...