The Israelites copied the worship of the calf from the Egyptians, who worshiped the god “Hapi” under the form of a male calf.
This in turn was copied from the Babylonians,who used the bull as a symbol of Nimrod. Not only was a bull chosen
because it symbolized might and strength, the Chaldee word
for “bull” also carried a double meaning. Nimrod as a king
was a ruler of the people, and the name for “ruler” is “Tur,” that also means “bull” in Chaldee. Thus the bull
was chosen to symbolize Nimrod in his worship.
That’s also a great explanation Dave Tesla. But the innermost (Chassidic translation of the “real” reason) is that weeks earlier at the giving of the Torah, the Jews saw a figure of the face of an ox on the angels carrying G-ds throne. They thought—good enough for G-d....good enough for us! And no human frailties! (they were disappointed that Moses chose (seemingly to them) to stay in heaven rather than come back and lead.