I think t-shirts and jersey are effective advertisements here in the States. I have a ringer t-shirt with the Southampton logo. Can't remember where online I found it. It was not easy.
Try going onto any kit or jersey website and try finding Southampton gear. Pretty much barren. Plenty of Arsenal, United, City, Chelsea gear for sure.
Also, in the states, it would be nice to get Southampton on TV now and again, but they haven't been on TV since week one with Liverpool. I thought this week it would happen for sure, being it's a Sunday game and an "attractive" opponent, but...
guess what? Chelsea v Arsenal is schedule at the same time. Guess what American TV is going to show?
You can still get every Southampton game streaming here in the States, it just takes effort and is not as "visible".