The purchasing process is the barrier to entry. Municipalities are required to run a request for proposal on anything over ~10k. The RFP gets awarded to a PE owned manufacturer that then takes 4+ years to deliver. The municipality is locked into the vehicles they don't have so a new market entry needs to simultaneously establish a track record that qualifies them for being awarded an RFP and be able to wait out the purchase lag. Not impossible, but easier said than done.

The Capex and Opex requirements to do anything in the US are THE barriers to entry.

Nobody has that kinda cash lying around, banks can't justify such high liabilities, and VCs are not interested in "stable", businesses.