Why was King Solomon a philander?

I think you mean a philanderer. Simply, because as a King he fell away from God and lusted after his 300 wives and concubines. Even though the culture allowed it, after all he was the King, this was contrary to God's teachings to have only one wife. Solomon began to worship idols also and all of this was abominable unto the Lord. Although, he had been the wisest man, he became wicked and foolish. Thus, we have another example of men called and blessed by God who fall due to their arrogance and foolishness.