Probably because the Jaeger program is mostly a failure, even in the movies. Only Gypsy Danger can ever do anything. All the new Jaegers get taken out immediately in the first movie, and in the second movie they suck too. I'm gonna go out on a limb and say this is a massive time jump of 20-50 years, and humanity lost to the Kaiju. Totally ripping off the premise of the final fight of Robot Jox, they find an old Jaeger that still has power, and they somehow fight without a maintenance crew to maintain it after each battle? Although it seems another Jaeger appears at the end of the trailer.