Because NXT doesn't have to worry about appealing to kids, and casuals, and selling toys, and appearing on Sportscenter and shilling reality shows and trending on Twitter. It's able to be a much more focused product.
You say that, but the main things that make NXT enjoyable (long-term booking, logical finishes, and great matches) are not things that fly in the face of "appealing to kids, casuals, and selling toys". It's easy to explain it away like that, but that's just not the case. NXT is just booked smarter.