When the newer classes of SSBNs came on line, with longer range missiles which could not be retrofitted in the older boats. As this happened the older boats were taken out of service - Hotels replaced by Yankees, Yankees replaced by Deltas. Unlike older tanks which can be stored and brought back to service relatively quickly (if properly stored), or even the aircraft stored in the boneyard, you can't "park" nuclear powered submarines somewhere, and then bring them back online. The USSR had limits in how many SSBNs they could have active at any time due to maintenance needs, staffing issues, and so forth so it is not like they hand a bunch of Yankees with shorter range missiles lying around ready to deploy as Deltas came on line - during the transition period the older SSBNs and their missiles were still needed for targeting against primary targets or stashing for later use. You could use SSGNs, both diesel (Juliett ) or nuke (Charlie) could use their missiles against land targets. The issue with that is that the usual loadout of missiles had, at most, one nuke the rest conventional. That could change, but then they are not available for conventional missions which are their primary job (anti-shipping). Furthermore the range of those missiles is relatively short requiring them to deploy well out of any normal area and relatively close to shore and land based ASW efforts. Finally any Soviet submarine could carry nuclear warhead torpedoes. Again the normal loadout was one nuke at most, like the SSGNs this could change but again this diverts them from their primary mission and the yield of nuclear torpedoes is relatively low, and the range is very short so basically the subs would literally have to be in the harbor to do this.
Another issue of using SSGNs/SSGs and SSNs/SSs in a nuclear strike role, basically arming them mostly or completely with nuclear warheads is that you've now lowered the threshold for using nukes - if the only missile/torpedo available is with a nuclear warhead then a sub may use it in self defense letting the genie out of the bottle too soon. Even if nukes are not used too soon, these classes of submarines have primary missions other than nuclear strike against far away targets. Their doctrine, training, and operational patterns for decades shows what their primary missions were: anti CVBG operations, anti-shipping, protection of SSBN bastions, etc.
I can't continue to state often enough hat the important factor is NOT how many nuclear weapons the Soviets have, it is how many delivery systems they have on the day the button is pushed. No matter what is done to tweak the system, 100% of missiles, bombers, SSBNs will not be available on button day. Some of the warheads fired will malfunction, some of the bombers that fly will be shot down or have mechanical issues and not hit the target, and even when everything works well every delivery system has a CEP defining accuracy, so some targets won't be hit in the sense that the warhead won't go off close enough to do the desired damage. When you are doing nuclear targeting, there are a bunch of mathematical formulae you use to take these factors in to account and come to a determination of what delivery systems/weapons need to be used (including how many) to ensure a given level of destruction of the target with a given probability.
In a nuclear exchange both sides know that the number of delivery systems they have won't be added to until well after the shooting stops (if then). If a target outside of Europe/USA/USSR is important (like an overseas base), then it is worth expending a warhead and delivery systems. If it is not so important, like Canberra, hitting it would be something to do later, maybe.