By the time of Koine greek, in general, it was much the same as today, but I always see the Ancient Greek pronunciation being taught, why is this? Is is it because most people learning koine in english tend to be Christian and learn it purely for just for biblical interpretation, not for to be fluent?
Much simpler reason. The teaching of Koine Greek is dependent on the local tradition for the teaching of Classical Greek: Classical Greek is more prestigious in language teaching, and is how most language teachers and linguists have been exposed to pre-Modern Greek. There was not a strong impetus for a separate tradition of pronouncing Koine in teaching to evolve: whatever the local convention was for pronouncing Classical Greek was simply carried over to Koine.
And that's not that absurd an outcome. It is practical—the local convention for pronouncing Classical Greek is going to be well known and widely taught, and is (most of the time) stable. It also avoids drawing an unnecessary barrier between teaching of Classical Greek and teaching of Koine.
As it turns out, Koine likely did sound much closer to Modern Greek than Classical Greek; but Koine was a system in flux (which is the point), and there was possibly register variation in the pronunciation of phonemes, precisely because it was in flux. That's a complication that's utterly useless for language teaching: do we need people to be pronouncing phis differently in Luke and in Mark? Do we have any certainty when phi switched pronunciation for a particular social group to begin with?
A stable system like Teaching Erasmian is simply more practical, however ahistorical it may be. Unless you're jumping into a timeship, meticulous historical accuracy of phonetics is not actually the primary consideration in teaching dead languages.