Yes that is a great question. I wonder why they chose 20 and not 100 or so. They are very focused on efficiency.

The 20 get chosen out of a pool that is larger, which makes it tougher to cheat. If someone would act up he can be voted out, since there are more witnesses to be voted to be the core witnesses.

I think 20 is still a decent amount of "miners". In Bitcoin there are only 12 mining pools and 3 of them have a majority of mining power. The 5k nodes we have in Bitcoin do not actively mine blocks. They still verify Tx and keep a copy of the blockchain but they are not the nodes satoshi describes in his white paper. So steem to me seems more decentralized than Bitcoin after all. Hower this might be wrong. I just find it very risky to have 3 entities that can control the network or could be compromised by an attacker.