I think it's mainly that there are other, more obvious problems to tackle before getting to STV etc. One thing at a time and all that.
Even those things are immensely complicated politically, because they would upset the existing balance between the parties, and in the current climate of extreme partisanship, whichever party is affected is going to pretty much automatically be against it. For example, if we switch from electoral college to national popular vote, it's pretty much guaranteed to give all future presidential elections to the Democrats, at least until GOP has a massive reform. Obviously, they're going to be against it. Similar issues arise with gerrymandering, and even such normally non-controversial issues as making voting easier (increasing registration periods and making the process easier, increasing advanced voting period, mail-in voting etc).
So I wouldn't expect any progress on that front until the present deadlock is broken. On the other hand, whatever new party will arise on the right from the ashes of GOP (or existing third party take over the spot), is likely to be in the minority for a while - and hopefully, that would motivate them to make electoral reform for a more proportional representation a prominent part of their platform. On the other hand, Democrats are likely to get more complacent and corrupt from a long uninterrupted stretch in power, and a few particularly nasty public scandals could be used to prime the voters for an upset election. So we'll get there; it'll just take time.