How do we get safety features before they're legally required?

My 2011 Honda Accord is a showcase of safety features that no one was forcing Honda to put in. 

Stability control wasn't required until September 1, 2011, but my Accord was built on March 28, 2011 and has it. So do a majority of vehicles as far back as the 2007 model year.

Side airbags weren't required until September 1, 2016, with FMVSS 226, but good luck finding a vehicle built five years before this that doesn't have them. Does the 2011 Accord have side and side curtain airbags? Yup, just like a majority of vehicles built as far back as 2008, and many from before then. 

Crash testing agencies played a huge role in getting these features in vehicles early. Ultimately, this ties back to consumer demand: who wants a car that has gnarly crash test results? Seeing the heads of the dummies slam into the barrier with deadly force in the IIHS side impact crash test was enough to put cars without side airbags at a competitive disadvantage, spurring fitment of these airbags. Vehicles where the airbags were optional provided valuable comparisons to consumers. 

The 2004 Honda Accord was one example. No side airbags? Dead. Side airbags? You'd walk away. The 2005 Accord provided side and curtain airbags as standard equipment. 

Uncle Sam didn't say you had to have stability control until the 2012 model year. IIHS required it for their lucrative Top Safety Pick rating from 2007. Automakers raced to put it in. 

Frontal airbags and anti-lock brakes were also installed long before they were legally required in most vehicles. All 1998 cars (that's September 1, 1997) and 1999 light trucks (that's September 1, 1998) had to have frontal airbags. and ABS wasn't mandatory until September 1, 2011 (it's an integral part of stability control). Look at this article from July 17, 1994. Most of the vehicles listed are '94s, and most have dual airbags. Most of those that don't have a driver airbag. Only 14 out of about 200 vehicles didn't offer airbags or ABS.