Because the folks who want to issue junk on the chain opt to go to specific miners directly to include their junk. These specific miners then outperform smaller miners due to the fees they receive in this manner — rinse and repeat this be behavioural pattern and soon we have centralization of mining. The reason why they go about this way is because if they just send their non-standard txs to the mempool of their nodes the propagation path for remain scarce because of the data carrier limit. This limitation feeds this behaviour.