Because the whole point of the 2016 operation (which involved support for lots of different, often diametrically opposed, positions and candidates) was to set up a situation where the US was even more politically divided against itself, weakened and ineffectual in foreign affairs, and set on a course to continue inflicting self-harm, and it obviously succeeded beyond Putin's wildest dreams, and there was no apparent need to interfere to change course evident in the run-up to the midterm. Plus, everyone was looking for it's so better to keep powder dry this cycle and leave 2016 seen as either a one time thing (or even, for the truly gullible, a thing that didn't happen) so next time there is a need, there isn't as much guard.