As a family of serial house renovators, doing it ourselves as and when we can afford it. The answer to your question quite simply is - money. Work is done as the money is available.
You stretch yourself to get the house you really want, it might be something special in your price bracket because it is so run down. Having spent every penny to buy the house, it takes time to get the money to do any renovations.
Sometime money is expected - like a regular bonus - but fails to appear, or redundancy happens. Also one person's essential 'do immediately' job is someone elses, 'happy to live with' feature'
Another reason is that quite simply, the purchasers are simply not interested in house renovation. As long as they have a roof over their heads, that is all that matter. DS inclines that way. 20 years ago, he and DDiL bought an immaculate but dated house and garden in a nice road. In 20 years, they have replaced the roof, kitchen and boiler and decorated once or twice. They still have no downstairs cloakroom and the bathroom is untouched, although they have bought towels to match and in the battle between man and garden, the garden has won.
The reason is that, apart, from having and bringing up children, they are a family that are always doing things, DDiL is part of a highly regarded folk group, as well as the day job, DS is a morris dancer, the children do scouts and dancing, and drama and a host of other things and there is lots of socialising, they are just far too busy for DIY and cannot afford to keep paying for people to do work and are not that bothered anyway, because a friend has called by and they have put the kettle on.
But they are happily married, the children are doing well, they are at the centre of a big social circle and 2 members of the family have minor but debilating medical conditions, which can lead to hospital admissions and minor emergencies. If they are happy with life and with the house as it is, that is all that matters.