OK my knowledge of tanks is limited, but since you asked:
Probably more like why wasn't the Panther first named Tiger B?
Sd.Kfz. is an ordinance inventory name, the term means Sonder Kraft Fahr Zeug which dissected literally means "special powered go thing", but really means "Special purpose vehicle", i.e. military vehicle.
The designation of a tank would be Pz.Kfw literally meaning "armored fighting vehicle" but really meaning "tank".
Panther is Sd.Kfz 171, Tiger (1) is Sd.Kfz 181, Tiger B (2) is Sd.Kfz 182.
Others more knowledgeable can now have their fun.