Ballotpedia has this very useful analysis of the regularly-scheduled state legislative elections. The study finds that in 42.6% of the races, only one major party candidate is running in the general election. This is even worse than the norm in even-numbered election years, when many more states hold regularly-scheduled legislative elections. Generally, in even-numbered years, between 32% and 37% of the races have only one major party person running.

Because there are relatively few minor party and independent candidates, this means that most voters this year have only one choice on general election ballots for state legislature. Legislatures dominated by Republicans and Democrats pass laws that make it very easy for members of those two parties to run for office, and (in many cases) very difficult for anyone else to run. Then, the two major parties, having achieved near-monopoly status for themselves, do not even use their guaranteed spots on the ballot to recruit and run nominees much of the time. And yet courts continually uphold restrictive ballot access laws under the theory that without those laws, the ballots will be too crowded.