Well I will try to explain at least what I think.
As you saw in the video you need 2/3 of producers in order to produce a irreversible block. In percentage terms 14/21 is the same as 140/210, but in the later you need to produce 140 blocks to reach this irreversible block with would be 10x slower. Also token holders would need to choose from a pool of 210 or so producers witch would take longer and harder to decide. So conclusion 21 looks like a good balance between decentralization, speed and consensus. My 2 cents.