I think they just ran out of money.

They might have also had the problem of series continuity...there's no book that comes directly after SC except for LB, which wouldn't make sense to do if they hadn't done MN yet. They may not have wanted to do a prequel for a TV show and interrupt the flow of the story.

I'm pretty sure it had to do with money, though.

I agree with MinotaurforAslan that the budget was a contributing factor. I also read an article (I wish I could recall it, but if I do find it, I'll let you know the name) that speculated the rest of the Chronicles were not produced because the themes "were too Christian".

A fourth reason could be that they never attempted them because the final three books are the least-known books of the series.

And a fifth reason could very well be because of political pressure within the BBC, itself. This series, as far as it went, has been a nice little earner for BBC and consequently for shops that stock it, such as Amazon or ABC bookshops. It says heaps that it is still possible to get DVD's without difficulty a decade later. But I doubt that in 1990 or thereabouts, with a cost-cutting government and an anti C.S.Lewis/Christian literati around, especially within the BBC, itself, that anyone would have foreseen this.

There is a sixth reason I wonder about. Did the people who vet these things decide that Richard Henders' performance as Rilian was a mite too violent for future adaptations as well? BBC in past years has done a terrific job in bringing to the TV screen a number of Children's Literature favourites including Alison Uttley's 'A traveller in time' and 'Tom's Midnight Garden'.

I believe too that they never were able to get the contract for all of them.

I think the later books are also fairly harder to adapt. Maybe that was one reason. However I have a feeling the main reason had to do with budget issues.

I agree that the rest of the books were probably just too hard to adapt given the BBC's budget.

I heard, somewhere, that they had in fact begun HHB bbc version, but had to quit. If that's true, somewhere out there is the only screen adaptation of HHB, sitting alone, barely begun.