Both Mothership and Scum & Villainy are entire games based around a single adventure type: survival horror and small package trade respectively. Traveller has been able to run both kinds of adventures for the past 50 years and many others. When you specialize too much, you breed in your own obsolescence. Mothership and Scum & Villainy are one trick ponies and get a lot of visibility at this time because the unpaid advertising for them is good. They don't have longevity.
I'm sure somebody said
Traveller was a one-trick pony back in the '80s. I've been reading
This is Free Trader Beowulf, and it's pretty clear there were numerous times when Traveller could have easily disappeared into the cosmos.
Earlier, you mentioned nobody plays these games in the wild. The next convention in my area lists several
Mothership adventures, but none for
Traveller or Scum and Villainy. At Start Playing Games,
Mothership currently offers 20 options,
Traveller 19, and well,
Scum and Villainy is just 1, but if you include the core system, you get 28.
Scum and Villainy may not be getting much playtime, but Blades in the Dark sure is.
My local game shop in a major metropolitan area sells both
Mothership and Scum and Villainy. They do not carry
Traveller. I asked about it, and they said it did not sell. They had tried in the past. FWIW, they are a successful hobby shop with several locations and a strong online presence. They also do seem to stock and sell through other Mongoose product lines.
Mechanically, both games have been discussed by game designers, reviewers, and critics, and have both been credited with significant contributions to the hobby.
Mothership has won at least 3 Ennies.
Mothership month on Backerkit raised $858,000 to fund 27 products, including $400k+ on a campaign setting. By comparison, the
Traveller Singularity Kickstarter raised `$224k USD. That is not a "one-trick pony."
HOWEVER, they are limited in what they do. I ran a year-long
Scum and Villainy campaign and played a few
Mothership one-shots. They were good games. I also found my way to
Traveller because I was looking for something that could do a little more. I can't imagine I'm the only one with that experience.
What's probably a heretical position is that I think there's a lot more similarity between some of the narrative rule-light games and Traveller. The 2d6 basic mechanic with degrees of success is very close to the core Blades and other indie systems. I also see parallels between Traveller character creation and the Blades/PBtA playbooks, in that both put constraints on the PCs.
But the bigger point is that I WANT to play
Traveller and get players excited about
it, but damn, that process is rough at times. I did not have that experience with those other products because they were much more user-friendly.