A very key understanding to Torah, which is really Genesis-Revelation, not just the first 5 books as the orthodox Jews wrongly believe, is to understand that this 66 book Torah is progressive revelation. In other words, one covenant and one law builds upon the other.
For instance, the Mosaic Torah does not do away with the precepts given to Avraham or Noah. Nor does the Renewed Covenant do away with the Mosaic Torah or Avrahamic Covenant or Noahide precepts. They all build upon each other and ALL are to be obeyed and kept.
With that said, while polygamy was PERMITTED long ago, this was primarily due to the fact that King David, Father Avraham, etc did NOT have as much revelation about the Torah as we do today.
This is scripturally obvious when King David will pose questions in the Psalms that we have the answers for today. He would ask YHVH about the nature of the future Messiah. David and the Prophets never got to meet YahShua in the flesh. They never got to read the gospels or the Brit Chadasha. This is why we read that the angels and prophets longed to see what the apostles were able to see. Of course, we now know through the revelation of YahShua as the Torah clothed in Flesh, the answer to that question.
As much as King David was anointed, he did NOT have as much revelation as we do about Torah, just as Avraham did not have as much revelation as David did.
Though the perfect wondrous Torah was given to Adam in the beginning, sin caused much of this knowledge to be lost, and especially after the Flood, Yah had to slowly work on restoring this Torah.
Because, through Messiah, we have such revelation of the Torah, polygamy is no longer permitted. At one time Yah winked at this cultural custom and even permitted it under Torah, but as more and more light was allowed to shine upon our minds and hearts through Messiah, the polygamy issue was done away with.
This is why Apostle Paul made it clear that to be a bishop or leader, one must only have ONE wife, because many of the pagan Greeks coming into the faith had multiple wives and so, though they could come into the faith and not have to divorce all their wives, they could not take leadership positions. Interestingly, the Hebrew culture had not practiced polygamy for several hundreds years when Messiah YahShua arrived on the scene in the 1st century.
YahShua also stated in the gospels in discussing divorce that "from the beginning it was not so". In other words, when the FULLNESS of Torah had been originally handed to Adam, the writ of divorce was not necessary. Now, that the FULLNESS of Torah has been restored again through the second Adam, we see that both divorce and polygamy are things that should NOT happen if our hearts are not hard. For from the beginning, Adam had only ONE wife, not many.
I hope this explains scripturally why Torah is still commanded for us to keep today but polygamy has been done away with once and for all!