Anyone can become a Bitcoin miner to try and earn these coins. However, Bitcoin mining has become increasingly specialized over the years and is nowadays mostly done by dedicated professionals with specialized hardware, cheap electricity and often big data centers.

If only 21 million Bitcoins will ever be created, why has the issuance of Bitcoin not accelerated with the rising power of mining hardware?

Issuance is regulated by Difficulty, an algorithm which adjusts the difficulty of the Proof of Work problem in accordance with how quickly blocks are solved within a certain timeframe (roughly every 2 weeks or 2016 blocks). Difficulty rises and falls with deployed hashing power to keep the average time between blocks at around 10 minutes.