From a mailing list post I never sent.
The probability of a miner confirming a transaction is directly proportional to that miner's hashrate (ignoring propagation-related effects, including accidental or deliberate selfish mining).
Users who choose to send their unconfirmed transactions directly to miners will likely put more effort into contacting miners with large percentages of total network hashrate and less effort into contacting miners with small percentages of hashrate. For example, if Alice gives a high-feerate transaction to 90% of miners, she has a 99.9999% chance of it confirming in the next six blocks.[1] If she puts in the extra effort to give the transaction to the other 10% of miners, she only raises her probability of confirmation within six blocks by 0.0001%, so the benefit is probably not worth the extra effort.
If large miners receive a better selection of transactions than smaller miners, they'll outcompete the smaller miners and hashrate will further centralize with large miners.
An independent network of transaction relay that gives every miner access to every viable unconfirmed transaction avoids putting smaller and more decentralized miners at a disadvantage.
[1] 1 - ( 1 - 0.9 ) ** 6