Because its not a major source of revenue and not primary income for road maintenance (swiss have stellar overall road quality compared to neighbours), and mostly intended for foreigners to not overuse swiss highways in summer & winter tourist seasons.

This can happen easily due to location of Switzerland. Its true that inflation is basically non-existent here, something about fiscal discipline and overall highly moral population not afraid of work (unlike france or italy for example), and thus also government. Overall country is low to medium tax place so vignette price is in line with that.

Follow a bit their politics a bit and their domestic affairs and you will quickly note how unusual that population and this country is. Best place overall for me to live and raise my kids, thats for sure.