There is also the fact that the best advertisement tends to be "word of mouth" or nowadays the interweb. Demon's Souls had very little PR but it is probably one of the best selling PS3 games due to word of mouth. Ports don't need much, cause just saying you'll make it, and when it will release, is enough for people to mark their calendars and talk about it. Just looking at the DL on the major consoles of ports is enough to show that not much advertisement is needed.

Granted, their are times it should get some better PR... Front Mission 1 for the DS was a port in Japan, but outside of Japan it was a first time release, so I was really surprised how little SE gave it any attention and not so surprised when it didn't sell very well.