For some reason, I just find that classical structure of a concerto (or sonata) very boring and predictable.
You mustn't forget that since Mozart's period the world of music had unbelievably changed. Mozart's piano concertos didn't seem in the 18th century as they do today. Simplicity and brightness where their idials.
However, Mozart did amazing thing that where very powerfull. He is brighy but yet very beautifull. Chopin adored him!
Chopin's magnificent piano concertos built the same way but are totally different. But I also think sometimes that may be using the old classic structure limited his freedom of expression. He always prefferd the unrestricted forms such as the ballades or the scherzi. But even them such as the nocturns and valses where written in a classic structure of A-B-A . or similar. So i don't really know. He may felt good in the structured forms.