This is actually a really good question, and it goes to the heart of the large backlash against Common Core. I can’t tell you how often I have dealt with snarky or enraged parents who vent about “Common Core math” and how awful it is. It’s even indirectly addressed in The Incredibles 2.
The answer to this is far deeper than just the Core Standards Initiative.
In the early 2000’s, Congress passed a reauthorization of the Elementary and Secondary Education Act of 1968, or ESEA. That particular version was more commonly known as No Child Left Behind. While the revision to the law was largely seen as a failure, with its focus on high-stakes testing and failure to raise scores, NCLB did provide something to educational professionals that they’d never had before: data. Mountains of data.
For the first time in several generations, educational researchers had a treasure trove of information to try and glean insights into educational psychology.
A number of reform initiatives grew out of the analysis of this slew of data, across all disciplinary content areas.
One of the major reforms in mathematical educational psychology actually had gotten its start about twenty years before the formation of the Core Standards Initiative in the late 1980’s. (H/T to Jered Wasburn-Moses for the correction on the initial research publication.)
This reform grew out of the understanding that students were able to learn mathematics by rote memorization of particular algorithms, but could not apply that knowledge to advanced mathematics. They lacked what researchers understood to be “number sense,” or the intuitive understanding that numbers are comprised of other numbers, and that numbers relate to each other.