C# ASP.NET MVC - Why isn't  immutability by default within the Domain (Business Logic) more common?
I have been working with functional programming a good bit lately, and have found immutability to offer a different perspective in regard to solving problems. I am considering using immutable classes by default in my future development. 

Aside from potentially a small performance cost, why aren't immutable classes more prevalent as standard convention in normal "good" design? Is there a down side I am not aware of with immutable classes (barring performance)?

I know immutability has been frequently discussed, but I had trouble finding anything specifically addressing why it's not used more than mutable classes. 
C# ASP.NET MVC - Why isn't  immutability by default within the Domain (Business Logic) more common?
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I have been working with functional programming a good bit lately, and have found immutability to offer a different perspective in regard to solving problems. I am considering using immutable classes by default in my future development. 

Aside from potentially a small performance cost, why aren't immutable classes more prevalent as standard convention in normal "good" design? Is there a down side I am not aware of with immutable classes (barring performance)?

I know immutability has been frequently discussed, but I had trouble finding anything specifically addressing why it's not used more than mutable classes. 