Search for past threads on the subject about which ones are "officially" available on DVD/VHS.
Mostly all shows on Broadway are filmed for archival purposes at the Lincoln Center Library. You can view almost any show there at the library with special permission. This is also discussed in other threads.
As for why more aren't "officially" released - it's mostly about money. I asked Lonny Price the same question - he brought us CANDIDE to television among other releases - and he responded "it cost $500,000 to produce a show onto DVD and we're lucky if we make half of that back." There just isn't a demand for it, at least not enough to make a profit.
But then you ask why PETER PAN, SWEENEY TODD, FOSSE, SMOKEY JOE'S CAFE, JEKYLL & HYDE, CATS, THE MAN WHO CAME TO DINNER, and other shows have a DVD available...and for that, I cannot give an answer because each case is different. CATS should be obvious. But something obscure like THE MAN WHO CAME TO DINNER? The only thing going for it in terms of marketing is the name Nathan Lane. Other than that, why would anyone outside theatre want to purchase it?
And then there's always the argument that theatre is meant to be experienced live and in no other format. I disagree with this mindset, because not everyone has the the opportunity to experience live theatre, so the next best thing is to watch it on your television screen, even if it does take away the purpose of theatre being 'live.'
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle