So the most promising technology NASA has funded for electrostatic displacement of dust in a way amenable to a space environment is called EDS, or an Electrodynamic Screen.

This is essentially a layer that fits over the solar panel with some electrodes embedded in it, patterned in a way that allows us to "walk" dust off the panel using a specific sequence of charge bursts. At first glance this seems like a silver bullet, however this technology has yet to leave the prototyping lab and make it onto a Mars mission for several reasons:

1. Efficiency. The presence of these electrode layers has been found to cut the power output of the panels by 15-25%. This is a no-go, until further developed.
2. Cost. Since this is very new, no industry has developed to marginalize the cost of making these. Since they are essentially still lab prototypes they are still very expensive to make, which makes allotting funding for them kind of tricky. Class B missions (like InSight) have less $$$ available, and Class A flagship missions (like the rovers) are important enough to have an RTG provide power. So at present, this technology is sort of stuck in the middle between function and price; at a certain point you have to ask, if the mission is so important that it *needs* to work long-term, why not just use an RTG? Especially when almost every previous solar panel based mission lasted decades without such a technology.
3. Safety/Reliability. A key point of an EDS is that it produces charge using voltages in the kV level. This runs the risk of arcing/damage at a certain point (Paschen's breakdown for the engineers). The breakdown voltage at which an arc forms depends a lot on the atmosphere, and we simply need further research into the composition of Mars dust in order to safely design an EDS that guarantees minimal risk of arcing.

TL;DR - It'll get there, it's not there yet.