Commissario
Oddly enough because of legal and money issues.
A lot of content was created back in the day when you might have expected it to be aired once, and maybe a repeat, so contracts were drawn up and signed with that in mind.
Some of it was created when you might have possibly expected more than one repeat so contracts were drawn up for that (with say an option to renew it more easily).
Some was created back when VHS was still new so contracts were done with that in mind.
Basically depending on when something was created there are a bunch of different contracts and to get an "over riding" new contract the BBC has to get a certain number of the key people involved in every contract/contract type to agree to it, if they don't they're in potentially serious trouble both with copyright, and with the unions/guilds who can basically say "you're messing with our members legal and contractual rights, none of our members will work with you until this is sorted". There are also some contracts (required by law at the time) that allowed "key personal" to veto their repeat or availability outside of the original contract which is why some things were not shown for years despite public demand (either a key actor or senior crew member who needed to agree to it, didn't).
The BBC isn't the only organisation to have these issues, they're just the one that is most obvious in the UK, ITV from memory has issues with a fair bit of it's old content from at least one of the Franchises, often because they lost the basic paperwork that proves what contracts were signed* and who was involved so in some cases can't even realistically start to get the "on demand" rights.
In American content, a fair number of things were delayed a long time in coming out on DVD because for example music rights might have only been agreed for "home video tapes", hence some at the time relatively recent series came out after long delays to try and get those rights, then had most of the background music replaced (in some cases even the opening themes).
IIRC the BBC spent a good part of the late 90's and early to mid 00's trying to renegotiate a number of their early contracts, both individually and then with the various trade organisations to agree a fee structure for them, and how to deal with instances where someone whose consent was needed by the old contract but couldn't be traced/whose estate was in limbo, from memory it ended up as something like they needed to get the agreement of a certain percentage of the people involved, and then put any fees in an escrow account or something like that.
Basically having access to the physical archival media is just the first part of being legally. and financially able to make it available to the public. You can have a perfectly good copy of something as the broadcaster but not have the legal rights needed to show it, or you may have those rights but the cost of transferring it to a current usable format might be astronomical compared to the demand and it might require time on equipment that has both limited availability and a limited remaining life span** in which case you may have to make the choice between something that is mildly popular and something that is not in demand but is historically important for some reason, or needed for something else (IE some now famous politician's first election win might trump an episode of random sitcom 2395 or a play that was aired at 2pm on a Wednesday in 1982, or the need to pull off a 5 minute segment from an old show for something new means the entire tape gets run).
*The various mergers and and buyouts of franchises meant that archives of paperwork were not always tracked properly, or even kept.
**There are various archival formats that are very much on a limited lifespan, not because the the media is dying, but because the playback machinery is no longer made, and the people that used to run them, repair them, or make the parts are no longer around. A lot of the broadcast formats for video from the early days required a lot of skill to operate, and things like the playback heads had extremely limited lifespans so they went off for refurbishment every X hours if you were lucky and potentially a fraction of that if there was a problem. Last I heard it was getting hard to even source "good" used machines for some of the mass produced "production" formats that were used at every small production facility, let alone the specialist ones that only the big broadcasters ever used..
