How do studios decide which movie (classic) to release on Blu-ray?

This is merely observation and some speculation, but I've noticed a few factors:

Sometimes the studios wait to release a few similar titles (theme, actor, director) at once. Often in box sets with the films also available individually at the same time or a few months later. Possibly not applicable at least as far as Forsyth is concerned.

Sometimes they wait for an anniversary edition. If the film was made 19 years ago, why not wait another year to capitalize on a 20th anniversary edition? As this year is the 40th anniversary of The Day of the Jackal, this doesn't apply unless they're banking on a 50th.

Sometimes the studios have a distribution deal, and not exclusive rights, so new deals must be made with the production companies / producers. You asked about the studio owning the rights, so this does not apply.

Some older titles need to be re-assembled from multiple copies of the 35mm prints to get the best possible copy which then need to be "cleaned up" (removing scratches, reducing grain, color correction from fading over time, etc) which takes a painstaking frame by frame approach in a few worst-case scenarios. Given the deep catalog of titles for the major studios, it is not a quick task to do this for all their older titles in a quick fashion. Especially since the studios are constantly in various states of production on the new films coming out now. A possibility.

Studio heads change, and personal feelings can enter into the decision to fast-track titles, also. The head of the studio that greenlit a film in the 60's is not the same as the current head who may hate the film or the producer or director and may slow things just as a vendetta, though I have no proof of this. It is extrapolation from how newly-promoted TV executives cancel shows that they "don't get." A possibility.

When I started this answer, I thought I had a better (or at least an) answer, but I initially misread the question insofar as the studios OUTRIGHT owning the films and have tried to edit accordingly. I'll voluntarily delete this answer if people think it should be downvoted.

Movies are often re-released according to a schedule, which can be dictated by any number of variables.

Often, whoever owns the rights to a movie lacks the resources to directly distribute the film directly, and as such will strike up a deal with a Distribution company. This is becoming more and more common as studios are becoming less vertically integrated.

Unlike first run productions, which are handled by marketing and exhibition (New Releases always have press junkets, premieres and 'hype' to support their release), 'Classic' or 'Old' movies have to adopt different release strategies, which makes sense when you consider:

• The title has already been in circulation for a number of years (a lot if its a true classic), so you can't pitch the title on excitement of the unknown. Chances are, if they're buying it, they've already seen it.
• The film is likely being pitched to a potential collector, Fan (in the Matt Hills sense of fandom) or image fetishist (if the release has been digitally altered/restored).
• The title's strongest marketing ballast is the pre-circulated meaning that popular culture has ascribed it to, so trying to manipulate what the film means to its audience is counter-productive.

So there's quite a specific Market to cater for, and the distributors have the advantage of knowing exactly who is buying their product, and don't necessarily have to actively sell the content.

Often, Classic movies will be released at an appropriate, pre-appointed time. It's always a good idea to re-release It's A Wonderful Life and The Great Escape at Christmas, for example.

Cinema Paradiso is this year's remastered, re-released Christmas Classic, and was probably saved for 2013 because it coincides with the films 25 anniversary.

Also, the restoration and preservation of Classic Cinema is an expensive business, not to mention the responsibility concerned with choosing what is appropriate for restoration.

In the UK we have the BFI (British Film Institute), which is a charity whose main occupation is this very purpose.

The BFI are also, however, a commercial entity, and as such they co-fund the restoration by creating deals with distribution companies to physically distribute the files (or film if its a special and very rare copy), handle the DRM (Digital Rights Management) and produce marketing material within cinemas.

The cinema I work at uses Curzon Cinemas as its distribution arm, and as they have a mutually beneficial relationship with the BFI, we receive a lot of 'remastered' work, which is lucky for me.

Multiplexes always receive the option to exhibit these films, but usually choose not to, and will instead play what they consider a more popular current release onto all their screens, much to the ire of Cinema Critics like Mark Kermode.

Different countries have different institutions dedicated to the restoration of old work, but this is simply the most common method in the UK.

As funding costs are largely recovered through exhibition, and eventual release on formats like DVD and Blu-ray, it's a sad truth of doing business that they will only commission their most commercially viable options.

HOWEVER...

What constitutes 'Commercially Viable' may well surprise you, as the BFI is run primarily by curators who have a legitimate interest in the movies they restore, and understand the difference between commercial classics and critical classics.

Often, the restoration companies will commission a 'film season' for a particular director, or even actor. This is often the marketing angle with which they are able to obtain enough fiscal stimulus to restore an entire 'collection'. It's usually this that has happened when, as you have pointed out, a number of films sharing a similar element are restored and released together.

Typically, in a DVD/Blu-ray store the customer will be unaware that the titles have likely already been released as part of an exhibition campaign in cinemas, and so the DVD release looks like a co-incidence.