Why SQL Server doesn't support foreign key constraints on views?
There's no fundamental problem with doing this, since tables and views are both relation variables in the relational model.  Foreign key constraints are just a particular subclass of general constraints, which standard SQL implements using CREATE ASSERTION so there's no particular SQL language difficulty either.

According to this Connect item (no longer available, and not archived) it's simply a matter of prioritization.  If you would find this feature useful, vote for the item and particularly add details of your use case in the comments.