From the whitepaper witness restrictions.

3. Must have a minimum of 5000 coins and a minimum weight of 10000.

With current supply it's only 69 350 witnesses possible, this excludes coins on exchanges and those who won't own 5000 or don't want to witness. Then you have bittrex whales that might want to witness with 500 000 coins. Will devs reduce witness requirement in the future
as more users come and prices goes up?

It will be better if Poloniex whales don't get involved because then there will be less then 50 witnesses. Sad