"Code.org sums up the situation nicely:
"Computer science drives innovation in the U.S. economy and society. Despite growing demand for jobs in the field, it remains marginalized throughout the U.S. K-12 education system."
There are many reasons for this. As you well know, teachers are already stretched pretty thin, and often it seems like there's just no bandwidth to add something new to a very full schedule. Additionally, some schools have few or no computers and/or tablets for classroom use.
But the earlier we introduce children to coding, the more comfortable they will be when presented with more in-depth learning opportunities in middle and high school. Also, early exposure to coding helps teach children how important it is to understand computers as the valuable tools they are rather than merely fun playthings."
Via John Evans