Why have electricity travel across the country at 1,150 kV?
The design of powerlines is a complex matter, in which many decisions overlay.
The Powerline Ekibastuz–Kokshetau is a relatively recent build, finished 1985. There were two more lines spawning off it, one towards Moscow which is now driven with 500 kV, the other was dismantled.
It is connected to a large power plant that was built at about the same time.
It runs a long distance through a relatively empty area.
One can assume it was the prototype project for the idea of electricity distribution on scarcely populated areas in the soviet influence sphere.
What would influence an electricity provider to build a 1MV power line?
Build a huge power plant (not happening often)
In an area with low population density (not many people complaining about the build)
Not having a distribution network in place (only happening in the so called 2nd world)
Needing the power elsewhere (Ekibastus Plant is 4GW, power line is 5 GVA)
Simply put, anyone else who might need a 1MV power line, had something else built before it was economically feasible to build 1MV lines. Seeing the Moscow branch of this particular line being run at 500 kV despite being designed for 1MV says something about that.
So, if a 1MV power line gets built again, it might first be in Argentina or Brazil. But only if they decide to build huge power plants in places where most of the electricity is needed elsewhere.
Also, a lot changed in power plant technology in the 20 years since then. Smaller plants are more feasible, solar and wind technologies are finding their place. Today, a town like Kokshetau would get a medium-sized plant, and be done. Megaprojects to transport electricity are not needed much anymore.
I suppose the power line is the quirk of a 5-year-plan, really. If so, it was meant to be the beginning of a massive power distribution system for the rural parts of the influence sphere. But before more could be built, the system collapsed.
Power is current times voltage (\$I\times V\$). The power loss in a wire is \$I^2\times R\$, so, if you increase the voltage by a factor of, say, 4, the current required will be reduced by a factor of 4 and the loss in wire will be reduced by a factor of 16.
I assume the power line is really long, so using higher voltage means thinner wire can be used. This is one of the main reasons why AC won the current wars - back then there was no easy way of stepping up/down DC voltage.
Basically there're two factors. As voltage gets higher the current gets lower and losses get lower and that allows for thinner wires. On the other hand as voltage gets higher better insulation is required everywhere - posts have to be higher (so that no discharge into the ground happens), distance between the wires needs to be greater, and much better insulation is needed in the transformers at the ends of the line. So raising the voltage reduces transmission losses and wires cross-section, but induces lots of problems with high voltage itself. That's why the actual used voltage is a tradeoff - high enough to not lose too much energy as heat and not too high so that the system can be manufactured and run.