For years they were a scrappy independent company slowly getting FAA/NHTSA approvals and variances but without major financial resources — I recall they were one of the early participants in Wefunder back in 2014. (To this day, at least from my perspective, I'd say they're the most notable company to equity crowdfund. But I might not be a typical observer.) https://wefunder.com/terrafugia

Then in July 2017 they were acquired by Zhejiang Geely, the owner of Volvo. I imagine that gave them the resources necessary to create a manufacturing line and come to market. (Still unclear to me whether the purchase is because they'd finally reached a point in R&D where it was market-ready so they sold or if the influx of capital pushed them over the finish line.)

https://www.terrafugia.com/terrafugia-inc-100-day-update-sin...

https://www.terrafugia.com/terrafugia-inc-to-create-130-new-...

Partially due to legal/liability concerns and the FAA regulatory process, bringing an entirely new aircraft to the market in the US is very long process. Just the various manufacturers that have been developing diesel engines for small single engined aircraft have gone through very long testing processes.

Testing for certification takes a long time. Also, adding to the fun: FAA cert is only good for the US. If you want to sell your aircraft in Europe, you need EASA certification. And so on for Canada, Russia, China, Brazil etc. Though many of the requirements overlap, it may still take additional years to secure certification from each country's aviation regulatory body.