Part of the answer is dividing programmes up into smaller chunks to attract a wider range of bidders.

The problem is that, even when divided up, they're still often "build and run" contracts which are less attractive to the smaller consultancies which tend to specialise in the "build" part, and are much less keen on the "run" aspect (with good reason, as it tends to be low-margin and painful).

And even then, fashions inevitably change and suddenly someone starts wondering why they've got 30 different tech vendors and need an expensive enterprise architecture team to glue their work together. Cue "why can't we just consolidate all this and get AtoS to do the whole thing?", and the cycle begins again...