Several reasons really.
Mostly it is that it just works really well for her and us as a family - she has always learned a lot at home anyway as she is a geeky type and we are a family that read a lot, have science equipment etc hanging about and who go to museums and galleries a lot. When she started school we all found that it massively reduced what we could do with her - we went from three museums/galleries/landmarks, nature walks, projects and so on a week to just having a tired and grumpy child who was spending all her days in one place. We travel between two places a lot, which basically meant all holidays were spent in the other place, even when that wasn't the best time to go.
She learns really well at home, and either the school couldn't keep up (unlikely) or she was just keeping her abilities to herself (much more likely - we know she definitely was hiding that she could read as the teacher was really surprised when she 'caught' DD reading when she didn't think anyone was looking)
She finds it very hard in a noisy environment with too much going on. She s only five - she has plenty of time to work on these skills. She also is still regularly wetting herself. She spends a good proportion of each school day as alone as she can manage. However, she is a different child in small groups - recently I hid behind a wall listening to her playing in the street and nearly cried when I heard her compromising, coping with other kids not following rules and so on - all things we thought she couldn't do.
The only school available (she was on the waiting list for 8) is Catholic, and we are atheist liberal socialist as a family. She was starting to say that God doesn't care about her and so on.
We have all the resources etc anyway, and not being tired from school will mean she can do loads more - over the next half term we will be gradually introducing group activities, so by Christmas she will be doing Manderin, two dance classes, HE social club, Rainbows, HE art club, swimming class and various HE trips (we have booked her on to groups going to a fire station, a cathedral and a museum activity) as well as friends for tea, playing in the street, the playground we live next to and so on.
We are all very excited - she thinks it is amazing 