why can't it be big ...? , because bitcoin has not been officially used as a currency, and many do not know bitcoin, therefore bitcoin cannot be perfect yet. If bitcoin can often make campaigns, it is likely that bitcoin can be a big mining
No the reason is not that it is not centralized but the reason is that miners do not get profit because of low price of bitcoin. When the price rises up miners make more profit because they receive big money for their bitcoin. Now the expenses are high and profit is less. If the price falls a bit more they will definitely stop mining. Not earning is better than losing money and time.
Yes that is right to say that the minors are currently not getting good profit from mining, I think that they should continue mining and should hold the mined bitcoin for some time, I hope that 2019 will be a good year for bitcoin and we will see bitcoin price touching 20000$ again, and at that time I hope that they will get a very good profit from there.