Because every time the number of coins gets 50% closer to 21 million, the rate of coin creation is cut in half.

Imagine starting on a 10km hike at a rate of 10km/hour... after 30 minutes, you will have walked 5km. Then, you cut your rate in half to 5km/hour... so, after another 30 minutes, you will have walked another 2.5km for a total of 7.5 km, so you cut your rate in half again, and so on. By continuing to cut your rate in half every 30 minutes, you'll continue to be cutting the distance to your 10km target in half - which means you'll never actually ever reach it (let alone exceed it).