Why did the KJV Bible translators replace the name YHWH in the Old Testament with the pagan word "God"?

If they didn't want to try and transliterate a Hebrew word (YHWH) in which the vowels could only be guessed (and thereby, they knew that the resulting word would be mere speculation), why not simply translate YHWH as "I Am That I Am" or just "I AM"? Was the old Germanic word that evolved into the English word "God" already so entrenched by 1611 (from prior English translations) that they figured that there was no point in resisting established tradition as to how people referred to the Biblical deity?