So it is a re-affirmation of someone else's conviction, rather than a fundamental bullish case that can _cause_ Bitcoin to rise? An example of fundamental Bullish case (in my opinion) would be like more quantitative easing, or (e.g.) Amazon saying they will accept Bitcoin for payment etc. 

But from what you say, it is more like I am bullish about Apple because Warren Buffet is buying it, which is rational, but not fundamental. 

spiderman

bitcoin_beginners

Why does More Miners Means a Bullish Sentiment for Bitcoin?

Because someone has the conviction that Bitcoin fiat price is going to continue going up. And thus spending some CAPEX in fiat to get the equipment and paying electricity for years in fiat as well, just with the hope of getting something more valuable out of it (Bitcoin). Which currently is only true if number go up.