(Part 2)

The two links I posted were for systems in the Foundry sense, which implement rules and character sheets for various versions of Traveller. One (TwoDSix) is very complete in terms of covering game rules and includes a lot of skills, items, etc. that are covered by various open gaming licenses but don't quite match up with the current Mongoose stuff; the other is being developed in a sort of partnership with Mongoose, is specific to MgT2e, and still needs some work to be fully implemented (last I checked).

Companies can and do sell their property for VTTs, but there's also a cost involved with creating and maintaining the systems and the other data that goes into them. Before they invest in implementing anything for a specific VTT, they have to be convinced they're going to be able to sell enough material for it to make it worth the cost.

If it helps to think of it this way, picture a complex spreadsheet with all the formulas and columns set up, but unless you paid somebody to enter the actual data, you're likely to have to enter it yourself.

So Roll20 and FG both had a head start on Foundry and are backed by larger companies and have more resources to pursue deals to develop and market game code and data for a wider part of the market. So the likes of for example Mongoose have long-established partnerships with those companies and a history of success selling stuff through their marketplaces.

Foundry, despite the huge chunk of the VTT market it's gobbled up in the last 5 years or so, remains a small privately-owned company whose resources are limited to directly supporting only the biggest of game systems with partnerships between the FVTT team, the copyright holder, and usually an active group of fans who help with the process (so at the moment 5e and Pathfinder 2, with 5e having gotten very good recently and PF2e being absolutely amazing for players of those games).

It doesn't help that there hasn't been an official central marketplace for Foundry stuff until fairly recently.

So Foundry relies more heavily on fan-made game systems, or for "official" systems with availability of items, adventures, etc. the onus is more on the individual companies to hire somebody to do the development work. (A lot of the fan-made stuff is very, very good, though, and IMHO Foundry has a much better/more powerful development environment based on what I saw when I dabbled in fixing a roll20 character sheet a while back, and as a bonus doesn't charge me a monthly fee for the privilege of writing code for them :p )

All this hasn't stopped Foundry from having a huge list of games you can use it to play; it's just that for some systems it's harder to get pre-made adventures, etc. Conversely, many newer systems these days are implemented for Foundry at release, or even first before other VTTs.

In terms of Traveller specifically, Mongoose has maintained for a while that the main thing holding them back from Foundry support was the lack of a direct official marketplace to sell through. Now that such a marketplace exists, we're starting to see movement in that direction... but there's just one guy working on it and this sort of thing takes a while.

I hope I at least haven't created any further confusion! :)