Because women, surprisingly enough, love bastards
A little off topic for this thread, but I actually liked Johnny. I'd say that he had a difficult personality, a bit arrogant and dismissive of many things, sure - but he was also a man of principles and one of those people who could see what's wrong with this world and who/what caused it. I think he had a lot of bitternes inside as a result of seeing that after his depart from life and during his absense the world hasn't changed even a bit for the better. I loved our conversations and was always excited when he appeared, so that we could continue to tease each other and exchange some thoughts. And later in the game we actually grew supportive to each other and became friends. Johnny definitely had goodness in him, but it was kind of burried under his bitterness, just like I mentioned. An interesting character and an interesting choice for having him in our head as a big part of the story. Quite unique approach too.
But I understand of course that not everyone turned to like his "companionship" and how he dominated the main plot. It's safe to say that we can expect the DLCs being "free" of Johnny.