Because the rule has a phase-in schedule:
10% of the vehicles manufactured on or after May 1, 2016
40% of the vehicles manufactured on or after May 1, 2017
100% of the vehicles manufactured on or after May 1, 2018
Consider the supply-chain and manufacturing timeline of an average vehicle. Most models are replaced with a new version every 6 years or so, with a mid-cycle refresh every 3 years or so, give or take. Take the Honda Accord for example, which was last updated in 2013 but is still being manufactured and sold in great numbers today. Honda obviously had notice this rule was coming. This means for the Accord they released in 2013 and would likely still be manufacturing into 2019, they needed to already have the designs and everything ready to go for full compliance back in 2012 probably. This is why it is prevalent today, even if not yet strictly required.
As for the race to the bottom argument, check out goodcarbadcar.com. It has some good stats. Pretty consistently, the cheapest most basic models are the ones that sell the highest. The margins are better on cars that are better equipped, but the sales are lower. There is money to be made on both sides of the market; I do not debate that at all that there is also competition with the higher-end features. However, the sales data shows that a large percent of buyers are simply choosing whatever is most affordable, which suggests they would go lower if they could. For this market, it is a race to the bottom and its the governments job to ensure that bottom isn't so low that its harmful to the general public.