This article/feature highlights the dominant Dutch Paralympic tennis team and provides some very convincing reasons as to why the team is so successful. The long of the short of it: they get a lot of support from the Dutch tennis federation, which does not distinguish between wheelchair tennis and able-bodied tennis. Also a factor: tennis the second biggest sport in the country with most towns have more than one club. (This does seem curious to me given the comparative lack of Dutch able-bodied tennis players.) It also seems that the Netherlands are much better than say, here, getting newly-disabled people into sports. (I do wonder though if that may change here given the high numbers of disabled soldiers returning from war.) But apparently disabled people there are highly encouraged to participate in sports and are given sport wheelchairs.
Sounds like the Netherlands could provide a solid model for other countries to emulate.