The reason is simple. I feel the supply of Bitcoin at pegged 21m will be too limited for the global economy.
There is around $37 trillion in narrow money.
There is around $73 trillion in all stock markets combined.
There is around $90 trillion in broad money.
There is around $0.5 - 1 quadrillion in derivatives.
If we ever reached the point of 1 sat = 1 dollar, then 21 million bitcoin would be worth 2.1 quadrillion dollars, more than enough to cover every market in the world and then some. If you still think 21 million isn't enough, then a far better option to increasing the market cap is to subdivide it further. Lightning Network already works in milli-satoshi.