I'm a Political Science guy, not so much a "historian" but my take is that in general our nuclear submarines are kept very secret as that is pretty much the reason they were created (as part of our swith to a "flexible response" doctrine"). They are "survivable" second strike options. What this means is that if someone were to pull a nuke on us, we are able to respond because they don't know where our nuclear submarines are. In addition, the United States is ahead of the curve on SLBM technology, while many other countries have the know-how to make ICBMs as they are the more common weapons. SLBM technology is very valuable as the subs are great "survivable" or second strike systems who can't be found (at least as easy as land based launching silos) and are crucial to national security, this is not a technology and capability we want to make readily available to our adversaries.
Where most of my rudimentary knowledge comes from is this: "The politics of innovative military doctrine : the U.S. Navy and fleet ballistic missiles" By Owen Cote
A few thoughts:
There is a lot about early, decommissioned ICBMs. That is released largely because they are considered obsolete. It is also released because that information is not a stop-gap to another state wanting ICBMs — there is so much information out there on designing basic ballistic rockets (which are dual-use), and the early-1960s navigation computers are so primitive by modern standards that nobody would pursue them, and the accuracies of those early 1960s weapons would require a hypothetical enemy to use extremely high-yield weapons anyway, and this is just not the path that anyone has taken since the 1960s.
Now the question is whether it is really true that there isn't a lot of information about early, decommissioned SLBM systems out there. I'm not sure — it's not on Wikipedia but that doesn't say much. Anyway, let's take it as gospel that there may be less out there. Why might that be?
ICBMs use rockets that are not dissimilar to those used in space programs. They are in this sense dual-use and this is why there is so much land-based rocketry information out there.
SLBMs, however, are exclusively used for military weapons. They are considerably more difficult to build than ICBMs because of the size constraints. The physical missiles themselves must be smaller to fit onto the submarine, and they have correspondingly lower payload weights. They also have diminished ranges. The US has managed to make SLBMs with pretty impressive ranges and extremely impressive accuracy. The Trident D5 supposedly has a CEP of 90 m, which is bananas. To put that in perspective, the Minuteman III has a CEP of 200 m, and the Chinese DF5 has a CEP of 800 m — you could drop a Trident on a target the size of the Pentagon with almost 100% confidence in hitting it with the physical warhead itself, much less its explosive effects (which would guarantee up to 10,000 psi over an area the size of the Pentagon, which is approximately the pressure felt at being 4 miles under the ocean surface).
That isn't knowledge you can just pick up at your local space program, or buy off the black market. Only a handful of states have managed to produce good SLBMs, and one can see that the Chinese and Indians in particular are still eagerly improving and upgrading their SLBMs, because of the key role that such a "survivable" technology plays in having a second-strike status.
OK, so that's the "logical," "rational," "strategic" explanation. Could there be another? Sure. Army and Navy are two different agencies. They have different perspectives on the classification of their technology. They have different systems for deciding whether something is "safe" to release or not. Do their rules always match up, one-for-one? No. So it's possible that there may also just be different cultures of secrecy here.
But I'm just gesturing in the dark, here. I don't really do missile classification policy — I do nuclear warhead classification policy, which is a little different, and has a different agency (the AEC and later the DOE) with its own separate culture (and even laws — the Atomic Energy Act governs nuclear weapons secrecy uniquely). So these are just guesses extrapolated from seeing how other policy decisions on classification have been made, and my understanding of the differences between ICBMs and SLBMs with respect to dual-use.