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.