Here are some things I think are the main reasons:
1) They want to learn to speak with the same accent as people they know. And there seem to be more Southern voices living overseas.
2) Expats and wannabe expats alike tend to choose Saigon to settle. Modern lifestyle, more job oppotunities, etc. So naturally learning with southern accents would be more of help.
3) I don't know all about the technical aspect of it, but I do think Southern accents in general sound softer and more fluid than Northern ones (I'm Northerner FYI). One reason is that Southern dialect only has 5 tones -- ngã is pronounced the same as hỏi. So it might be less of a pain for them learners to learn it.