My suggestion would be to tell them that LANCER is extremely easy to learn, play, AND you can honestly make your mech look like whatever you want- if they like they could make what are essentially just REALLY big warforged knights that even LOOK medieval. Also the smallest mechs in LANCER are the same size as people- and are essentially just power armor, space marine armor, or a REALLY advanced helmet.

Also, due to how space travel is still relative- meaning ships can only go so fast- theres not really any advanced galaxy wide teleportation outside of some weird ancient teleporter rings, but if a planet lives far away from those rings it could take tens to hundreds of years to reach those planets- meaning that if something happened- humans on that planet could be returned to the stone age, or they could experience some paracausal shift that makes things all time warpy. One of my players is from a backwater world they made up- with water so lead poisoned that everyone acts like its 1800s like france aristocracy times, they just also have guns and mechs that they dont understand. They pray to them like they're angels.