I'll jump in on this one.
Right now we're coasting on the Sputnik era science financing, and slowing drastically in both the sciences and medicine. Funding of research is way down.
The number of science PhD graduates is also diminishing, due in great extent to the costs of education. During the Sputnik era there was no such problem, there were plenty of scholarships, research and teaching assistantships. Not loans that took most of your working life to be repaid.
I gave a guest lecture on invasive plants a few days ago. To environmemtal and civil engineering grad students. I had to explain what a dandelion was! Not one person in the group was American or European. The majority were middle eastern or Indian.
This type of thing does not bode well for the future of American science and engineering.
Yet another factor: textbooks in pre-college biology are awful because of two things. One of the two is directly laid at the feet of militant creationists. A lot of biology can't be properly explained without evolution and the book marketers cater to that group in order to get into some of the more influential districts.
An indirect result of that is that many really good biologists can't be bothered to get involved in writing textbooks that in their minds will be full of mush.
As a former college prof, I would suggest that there are many reasons: cultural distaste for science, inclination of student (and teachers) to be lazy, lack of funding (for teaching, not for school systems), lack of policy (nationally, locally), etc.
I personally taught in two different public schools for about a year and a half, and my personal experience was that there are a few good, knowledgeable science teachers out there and a whole bunch of ones who know little about science. I think there really is a shortage of qualified people who want to teach. There are, I believe, several factors causing this situation.
First and foremost is the state of university education for teachers. Prospective science teachers are required to take surprisingly few science classes, and a whole lot of education classes. This strikes me as bass ackwards. We should have teachers take a bachelor's degree in the field in which they wish to teach. Then, they could take a semester or two of education courses (or do this during the four years of their B.S. in addition to the B.S. coursework) and finally move into their student teaching. A firm grounding in the field is lacking in many educators. I think many of today's students who have some interest in science are attracted to science education programs since these tend to be less difficult and rigorous. Unfortunately this leads to a lowering of the qualifications of the average science teacher.
Secondly, (and the main reason I no longer teach), teaching in the public schools today involves very little actual teaching and a whole lot of administrative and classroom management type activities. Student discipline is difficult in today's schools. I don't know how it was for you in school, but if I got in trouble at school, I was in trouble when I got home as well. Now, it's relatively common for a parent to completely deny that their precious little son or daughter did what the teacher says that they did. Without parental support, and often without the support of school administrators who fear that the parents will make trouble or even sue if their child is held accountable for their actions, it's difficult to maintain any authority as a teacher. There simply is too often no consequence for student misbehavior. In such an environment, educating children is difficult, and many knowledgeable teachers (including me) just give up and move on to other jobs.
Thirdly, in the 1950's and 1960's, after the Soviets launched Sputnik, there was a great push to get qualified people into the teaching of science. In many cases, prospective science teachers had their college tuitions paid for in order to encourage smart people to go into science education. This has led in recent years to a fairly large group of older science teachers who are nearing retirement, and are not so motivated as younger teachers. These are the tenured teachers who are holding on until they get to their 35 or 40 year mark and can retire. I taught in 1997-1999, so this issue is probably on the decline because of attrition.
Finally, there is the issue of compensation. In many areas of the country, a bachelor's level scientist can earn 50-100% more than a high school science teacher. I personally feel that this is less important for explaining the lack of good science educators simply because a person who becomes a teacher based primarily on the money is likely not to be as good a teacher as one who enters the field out of a motivation to help students. Still, there may be many qualified individuals who want to teach, but choose an industrial R&D position instead based on compensation.
The lack of emphasis on science in public schools and in the country in general, I think, comes from a generalized dumbing down in the school system, which has led to an overall less knowledeable population. The school system is more concerned now with promoting an unearned feeling of self esteem in students rather than in real achievement. A good science education is rigorous and difficult, and we can't teach our children something that they can't master completely or they might feel bad. Therefore, we just water down the science curriculum to the point where most of the students have no problem with it. Only problem is that they then don't really learn much and we end up with a scientifically illiterate population.