Why is the main reason that people sell their miners?
I think the only is for obsolescence of miners, that is to say, that miners don't make profit, is this is true, other people can make them more productive?
as my knowledge to the miners sell the old mining rig because he/she want to upgrade and set up new mining rig, and make it to improve and get more profits for long term investment.
Of course! It's for profit and there's no profit in long term inefficient btc miners. Gpu's now carry for several years depending on model but that's for another thread. On average most Bitcoin rigs have about a year or just over to stay profitable from release.
If it weren't for power costs and btc price/difficulty no one would ever need to sell.