Why are whales not grouped in fishes?

Whales belong to class-Mammalia, while fishes are included in class-Pisces. Both are aquatic animals as well as chordates.But their classes differ because
(i) Whales are warm-blooded animals, while fishes are cold-blooded.
(ii) Whales respire through lungs, while fishes respire through gills.
(iii) Whales possess mammary glands, which are absent in fishes.
(iv) Whales have four-chambered heart, while fishes have a two-chambered heart.