Why would a secondary school NOT offer triple science GCSEs?

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A friend's DD is in year 7 and they've discovered that the school doesn't offer triple science GCSEs only combined science GCSE. The school does have a have a sixth form and offers biology, chemistry and physics A Levels. This is a big, well funded school that offers a range of different GCSEs and A Levels plus a lot of vocational qualifications.

Why would the school only offer combined science GCSE, and does it matter if the DC wants to go on to take science A Levels at the sixth form?

The dcs school offers triple science but only to the top 30 kids in the year - I’ve no idea how common that is, but the bulk of them will be doing double.
It depends on timetable.
Often, but not always, the schools that offer triple to the top X number will teach separate sciences in the same curriculum time as the rest of the year studies combined.

Where triple is offered as an option subjects the class(es) taking triple will have the 3 sciences split over the timetable covering combined science plus one option column.

Not necessarily, and the separate sciences are offered at both foundation and higher level - although the majority of those entered will be taking higher, because most schools only allow/encourage abler students to do triple.
The same can apply with subjects like Latin, which are more often taken by high achievers.

I don't know how they work things statistically - if they know the prior attainment of the cohort taking a particular subject, then that could be used as a guide to the range of attainment that might be expected in the GCSE. Or if the combined chemistry paper is a subset of the questions on the single-subject chemistry paper, they can use that to make sure that similar scores on the overlap correspond to similar overall grades.