Are any of the other non-2d6 systems that are not D&D any more significant commercially or non D&D D20 systems for that matter. It seems to me that D&D has the market share but then it has had a massive PR campaign (and a movie).
On this factual question, yes: there are several systems which are considerably more commercially significant than 2d6, if you want to restrict influence to that narrow definition.
I'm keen not to rehash this discussion
again, only a short time after a bunch of us went through it in detail, but on non-D&D D20 systems, yes: you can find non D&D d20 systems that are
massively more commercially successful than Traveller. Pathfinder (and its adjunct, Starfinder) would be the glaring and dominant example, here: it never beat D&D (despite rumours during 4e to the contrary, which were later disproved) but it was a genuine competitor for a long time, and continues to outsell Traveller by a vast margin. This is tier 1.5.
The next tier down would be BRP, and especially Call of Cthulhu (Runequest has faded). CoC sold 200,000 copies in Japan alone, and has sold in the millions worldwide over its life. Leaving aside the commercial criteria, it has also been phenomenally influential on the wider hobby, and has contributed hugely to the growth of interest in Lovecraft's work. While Traveller has faded since its glory days, CoC thrives (despite some
very bad business decisions made under the influence of
vast quantities of medical-grade mary-hoo-hoo by certain managers of the product, as Sandy Petersen has discussed since).
Then you have systems which have faded for various reasons, but whose historical sales remain larger than 2D6 Traveller. Dice pool systems like Vampire, the Masquerade jump out here.
I don't know sales figures for PbtA games, although they are clearly influential in non-commercial terms, judging by the range of successful games released using them in recent years. Not my thing at all but meh.
Then you have Traveller and similar games. Thus the thread about "how do we make Traveller more attractive?"
If you want more specific data, there was a long discussion a while back where I posted several charts on sales figures, usage on various VTTs etc which search can help you find. It may even have been in this thread vOv 2D6 and Traveller consistently came quite a long way down each list.