This begs the question about what uses the monuments were put to. If a large megalithic structure was either a focal/meeting point, ritual centre or a way of saying "this is my territory", then that may well be the case. However, if the circles were used for scientific purposes (?) and barrows for interment/excarnation etc, then it would be difficult to see the one fulfilling the functions of the other. In addition, there are plenty of areas that have both (and other mouments - round barrows, standing stones) in close proximity- Avebury/West Kennet, Merry Maidens/Tregiffian, Rollrights/Whispering Knights being the first ones I can think of!

In any case, there is evidence that a lot of the long barrows in the Cotswolds were abandoned or blocked up at about the same period that the earliest stone circles were being built elsewhere in the country (for example some of the blocking material incorporated pottery from this later period).

At a guess, it's no doubt a "combination of factors" (always a good standby!) such as availability of material, local population and beliefs and the way the landscapes have been altered from the middle ages onwards.