Because Barr has a duty to prosecute, not to engage in political discussions. Trump had advisors for the latter, such as Navarro.
If any new intel came up that Trump was privy, but Barr was not, then Trump was to have Barr briefed and Barr in turn had a duty to act.
The main thing for you to learn was Barr was not Trump’s political advisor, he was a Cabinet Officer, approved by the Senate.
That said, Barr had a duty to brief the President on the matter of Hunter Biden which he did not because he thought he could be insubordinate and disrespectful, thereby carrying favor with the Democrats. This is most likely due to the fact that Barr knew Trump was viewed as an outsider and the DC Class had it in for Trump.
Except this was not a political matter. This was a law enforcement matter - one that was of the highest magnitude since it directly involved corruption at the highest levels of government and directly affected US foreign policy. That's what you need to learn.
Do you think for one nanosecond that say, Eric Holder would have refused to answer to Obama about such an investigation....in addition to stonewalling said investigation? The AG answers directly to the president. The president is his boss - period. Unless the President issues an order that is illegal (and this clearly was not), then the AG must obey that order. If he doesn't, he needs to be fired on the spot.
Not that hard to make the case that if Barr had done his job
Ukrainians wouldn’t be dealing with being slaughtered by Russia right now.