they tend to be the only ones available as shared ownership, which is all I could afford. (a flat, not a house).
They tend to be in cheaper areas of cities and towns, which is more affordable.
They need less work done on them, which is more affordable for new buyers ,or less daunting for those who don't know enough about DIY etc.
They are less likely to need complex decisions made after having the survey done, which again might be overwhelming or daunting from people who don't know how much to compromise on different aspects.
They have smaller gardens etc often, which can make them cheaper.
They might come with additions like white good included, which can be cheaper.
The rooms are smaller and the plots closer together, which might be cheaper.
They might be more likely to come with car parking, depending on the area, which can be more convenient.
They might be what is available to buy in popular areas where older houses don't come up often, or not for reasonable prices.
They are often clean and smart and ready to decorate in the way you want, and might be more eco-friendly.
Don't forget that for a lot of people, living in a new build or on a new build estate doesn't mean that it was necessarily their first choice, but what was arrived at after compromises on various aspects.