Because you are merging Monero, a coin somewhat distinct for a clean launch, with a coin shrouded in mystery with a lot of suspicious manipulation around it (80% mined when released to the public, vague allusions to use in the darknet despite no real evidence as such, an incredibly polished pool miner (Minergate) within days of launch that just happens to charge 10%, team separated from the Cryptonote team for unclear reasons, etc.). That hurts the marketability of Monero. I highly doubt that Bytecoin will ever take off as it will never be able to shake the 80% mined problem. Look at how much trouble Darkcoin has and that was an order of magnitude smaller ninja-mine. 

Monero also doesn't need the Bytecoin network to secure its hash as it is already on track to surpass the Bytecoin net hashrate. It would essentially be linking Monero to a coin that will likely become increasingly smaller relative to Monero for no real advantage.