They say it's because they felt restricted in what content they could do for each pack. When they come up with ideas for every pack they probably decide on a couple of things and then maybe some features get scrapped along the way because they feel that they will not work or other features get prioritized. They wanted to have the ability that cc creators have where they can just make a bunch of items based on a niche theme.
The pricing is probably determined by EA and Maxis, based on how much effort they put into each kit and what they think people are willing to pay for them. The three kits will probably require a similar amount of efforts from people like artists and programmers. Dust the bust kit have two gameplay items with a lot of gameplay which probably required a fair bit of programming, the country kitchen kit required a lot of work to make all cabinets and benches work together and the throwback fit kit required a lot of work from their artists.
That's my guess anyway.
