Mine offers a boatload, but has eliminated Arabic and Italian over the years. But to answer your question, like anything, it comes down to money. Several boarding schools have offered Russian, Japanese, and Arabic in the past, but have discontinued them as enrollment declined. The 400 student schools just never warranted adding additional languages. It’s hard to justify a teacher’s salary when there are few students to teach. I am told that Russian and Japanese were “hot” in the 80’s and then drastically cooled with the fall of the Soviet Union and the Asian economic crisis. Arabic, hot for a while, will befall the same fate.
French and German, IMO, are carryovers from the days when prep school students did their Grand Tour of Europe. Additionally, for doctoral students, it was, and may still be, a requirement to have reading knowledge of one or more of these languages in order to read scholastic materials; now most of the key works are published in English. My dad days that in his prep school days, the vast majority of students studied French or German. Unfortunately, enrollment in these languages is also diminishing, and some schools have eliminated one or both of these languages.
The other thing to keep in mind is that Russian, Arabic, Chinese, and Japanese are not the easiest languages for an English-speaker to learn, and high school teaches at a much slower pace than college. So after 4 years, a student really won’t have that much to show for his/her efforts.
Once again, in these countries, English is quite important. The desire to do business in Czech Rep., Slovakia, Hungary, etc., does not require a knowledge of German. I have yet to be in a central/eastern European country where the inhabitants preferred to communicate in German over English, although there may be generational differences at play.
Portuguese is another language that has been mentioned here. Yes, there are a lot of speakers of Portuguese - almost as many as German, French, and Italian combined. However, they are all concentrated in one country, with little opportunity to practice in the US. I hardly think a 14 y/o has career plans honed so sharply that s/he knows that s/he wants to do a start-up in Brazil. If a student really wants to learn Portuguese, s/he is better off taking Spanish first, which will provide a solid foundation for Portuguese.