Bumping this because I think I've solved part of the mystery. After doing a lot of testing, I still can't figure out what - if any - trigger there is for the Dread Lords showing up, but I have determined that (at least in TOA; it might have worked differently in DA) there actually is a cut-off point for their appearances. It's not related to tech or exploration as such, but seems to be when the last habitable planet is colonized: unlike Vegans and other pop-up minor races, who seem to be able to appear pretty much anywhere, the Dread Lords need to spawn on a previously-habitable planet (whether it's extreme or not doesn't seem to matter). So if all planets are colonized rapidly on Suicidal, then it would probably be unlikely for them to show up.
I'll have to test this on DA, though, because I know the event was changed between the expansions.