Iirc, they were a part of Theryon Wars planet. Since it's no longer in development, I think it got put on the back burner. I remember Kim saying they were working on it as well. Along with Cyrene, but not 100% sure on the last one.
Maybe now that we have creature control mobs, it will be a part of that?!
It would take a substantial coding effort to create vehicles with anything other than no wheels or 4+ wheels. Looking at what other things we have on our road map I think its safe to say that mechs wont be introduced anytime soon.
The ones seen sitting on the flight deck of motherships and stations also implied that they would be present in the Calypso sector of the universe.
They would have been a great asset to colonists in the struggle against the robots of Akbal-Cimi, but I'm sure that due to regime changes within the Imperial Government, further development will not be SEEn.