Because they are trying to incentivize EV adoption not smaller vehicles. The top selling segments are SUVs and trucks. If you want to tackle emissions it makes sense to go after the popular vehicle types.
Not just that - top selling or not, you want to remove worse emission vehicles since they contribute a larger amount to emissions. One truck replaced with an electric version is effectively equivalent to 1.5-2 sedans, in terms of emissions per mile.
The Chrysler Pacifica PHEV has a <40 mile electric range. But vehicles like the Mini Cooper SE, all of Hyundai/Kia's EVs, and Tesla's standard range Model 3 and Y are missing out on the subsidy while gas guzzling minivans rake in the dough.