Saving the grass surface on the 2 show courts (that's my thoughts anyway)
That's my theory too. They stop using the outside courts later in the tournament, or at least there is no big deal about shifting between outside courts if one gets damaged.
They need to keep Centre Court and Court No1 in good condition right through.
I think it would be good to find a way to have more women's matches on Centre Court, but a lot is down to scheduling and how many sets you can squeeze into one day.
The era of 'the big four' has made it harder to demote their matches, especially as one is home grown, but I think every now and then they could try having three women's matches and one men's match on Centre. That's a theoretical 14 sets of tennis, rather than the usual theoretical 13. Maybe they should do that on the Tuesday when the women's defending champion kicks things off and matches are likely to be one-sided. I'm not sure if they'd risk putting three men's matches on a show court, especially one with no roof.