What do you mean noone followed the mosaic law?
I cannot possibly take you by the hand to that conclusion. You have to be willing to find it and learn it for yourself.
I can start you out.
Archaeology offers a controversial glimpse into how the Hebrews lived in the days of David and Solomon that is contrary to the Bible.
Even the Bible shows that after Solomon, the Hebrews were still worshipping other Gods.
YHWH was one of many Gods and the canon of scriptures containing the Mosaic law did not exist. Maybe some of the writings did, but not in any canon yet until during and after the captivity to Babylon.
Back up a bit: The Ten-Tribe Kingdom and the Two-Tribe Kingdom never ever were a Twelve-Tribe Kingdom. Never. David and Solomon may have ruled over small tribal kingdoms or even a large kingdom, but it was only the Two-tribe Kingdom. Very controversial is the idea that Josiah introduced the idea that all these tribes were under one rule at one time, this so that he could gain territory whose people he hoped would rather return to their (bogus) historic past than become part of Egypt's expansion. Josiah "found" the lost laws that nobody was living under before his time. The Bible shows that the people did not live up to that law very well after Josiah "brought it back" (euphemism for "invented it.") Many probably lived by some of it then.
The Hebrews that left Babylon for Israel and the people that remained were not under a true rule by God or a Hebrew king and the Mosaic Law was not in force upon them. Oh, they may have lived by the festivals and the principles (like Christians do today) and many probably followed the dietary restrictions but they did not go to all the extremes within the law.
Both in the time before the Babylonian conquest and after the return, necessary work was done on the sabbath and farmers rarely left a portion of their field for gleaning. They certainly did not refrain from growing according to the 7th years and jubilees, nor did they likely set slaves free in those years. And Hebrews, like other cultures, slowly lost their multiple gods.
Probably the closest people got to living under the Mosaic Law would have been priests leading up to the beginning of the Common Era. But that was a showy display for the people and that's a violation of the Law.
Again, this is all controversial and I will get arguments from plenty of people. Don't believe me. Fine. But take your spiritual journey for yourself.
If you want to disagree, fine. It's a big part of my spiritual journey.