You are greatly overestimating the amount of mechanical force an EM field can apply to electrostatically charged martian dust. Even at the kV level, an EDS panel cannot "pop" dust off. Nor does charging the panel allow dust to become mobile enough to passively slide off without further EM interaction, even if the panel were angled.

The way an EDS works is by shifting the dust by alternatingly charging the patterned electrodes in the screen. Because EM field intensity falls off with the square of distance, this pattern is required to keep the dust close to the charged electrodes.

And all this still ignores the issue of Paschens breakdown.

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.

I work internally with some of the Mars teams at JPL, and these are the best explanations I have:

1. Budget budget budget, many of these missions have a cost cap which can't be surpassed. Every additional thing adds system complexity, and ultimate ends up costing money to design/build/test. Even if there's an amazing idea floating around, if it's not in the project budget it's a no go. This includes solar panel dust removal, weighed against the expected science return and lifespan of the mission.
2. Martian dust is electrostatically charged, so it "sticks" to the panels rather than sits on them. This complicates removal.
3. Brushes are abrasive, and so is sand and dust. On top of the weight and mechanical complexity, point #2 means that you'd have to basically scrape dust *into* the super delicate solar panel to remove a marginal amount of debris. All said and done, wipers/brushes are not effective for this reason.
4. Blowers and fans, see point #2, especially relative to the amount of force such a device could apply as compared to a wind clearing event like atmospheric wind. Additionally, Martian dust is so fine that attempting to blow it away with fans/blowers sort of causes a floating cloud that gets sucked back into the fans, essentially turning it into a sand blaster.
5. There are methods of generating a burst of electric charge to displace the dust, but these are very expensive, very complex, and have not be fully investigated yet.