I think 16 million Bitcoins were mined by 2015 when Bitcoin price was $1000.
16,000,000 x 1000 = $16 Billion only
How did 16,500,000 Bitcoins got a value of $350,000,000,000 ??
Firstly there were 15 million bitcoin in circulation at the beginning of 2016.
Secondly, the market cap of bitcoin has never reached $350k, nor has bitcoin yet reached $20,000 (it did come very close)
Thirdly, 16,500,000 times by $20,000 is $330,000,000 not $350,000,000
Finally to answer your question. The price has increased because there has been a big increase in demand, bitcoin has had a lot of exposure in the last years and has grown as a result of that. Price is simply determined by supply and demand. Given supply hasn't greatly changed since the end of 2015 it's easy to realise demand must have grown a lot.
