Well... I am with seatta on this one, the 50s-70s are a particularly difficult era, asking for a lot more skill and knowledge, thus being more challenging.
A game running for 1950 to 1979 is no big deal either, because the difficult transition happening between those scenarios is skipped.
There are plenty of open GWs, and one of them is in the late 80s now, so whoever was wishing for a 1970-20XX game could have joined that GW 10 game years ago, or wait for the new GW to pass into the 70s era.
It's absolutely not impossible to start late in a game, because some people fail to perform that transition, or drop out for whatever other reason.
The most of the public demand states that people want a 1950-2030 game. The demand for a 1970-2030 game can be satisfied with the exact same 1950s scenario as well, because there are always spots opening up, and with the new "max-player-number-system" the amount of players allowed in the GW will increase by a few dozen anyways, meaning there will be enough space to join the GW. Also with the number being relatively small in the beginning, there will (or at least should) be plenty of available airports as well.