I guess the point of the ski speeder was to buy them time to send a help message and for someone to arrive. For someone to arrive, they would need to be in the cave for days. The FO knocked down their door in about ten minutes. The ski speeders did nothing but get people killed. Even shooting at the walkers with guns did nothing but get more peopled killed. Luke had to show up to buy time for the Falcon to load the last twenty people on board. There were a lot more people before they decided to fight to buy time. Why not just stay put and find an internal way to barricade themselves in and play Home Alone with the FO?