It's not about being hard. It's about cost. It amazes me on how few people understand this. How can you afford to sell a commercial application of justify a new LOB app if the cost is 3 times what it used to be to build?
It doesn't have to be. If you stop worrying about themeing and the more 'advanced' aspects of WPF you can knock out a quick interface in next to no time. I think, however, the temptation is to produce significantly better UI than you would with WinForms, simply because it's a lot easier to see that as a viable option.