Because you earn much less when using programs like Nicehash. We do know that it switches to the most profitable coin at the moment and mines it but it still is less profitable.
I've tried it for one full month and totally regretted it. I usually mine Zcash. When I was using Nicehash I got much less plus I even missed the opportunities of earning more how ?
Well when you mine a specific coin, there's a chance for its price to rise. That's the time when you should sell.
And the difference is not small. It's significant.
For example throughout September, the price was less than $200. And then it spiked up to almost $400 at the end of the month (sept 28 if I'm not mistaken.) That was simply an amazing chance to get almost the double of what you usually get.
If you were using nicehash at that time, you wouldn't have benefited from this rise as btc price didn't rise to its double