Question

why are some fungi grouped under fungi imperfecti

Answer 1

One of the classes of fungi is deuteromycetes also known as imperfect fungi.
This is because this class contains only those fungi which reproduce either vegetatively or asexually through spore called conidia.
Later if any member of deuteromycetes is found to have any linkages with sexual mode of reproduction the it is moved to other classes like ascomycetes or basidiomycetes.

Answer 2

Only asexual and vegetative stages of the fungi may be identified. So it is placed in fungi imperfecti until it's sexual stage is identified.

Once it's sexual stage is identified it is generally moved to ascomycetes or basidiomycetes.