Its not a losing case, because there is no losing case. Trumps entire defense case could be “chicken chicken chicken” and he would still be acquitted.
What is happening is that the Trump is (correctly) viewing this case purely in terms of public relations. His only political asset is that he has millions of rabid followers who would literally die for him, but only if he doesn’t show any weakness of compromise with the enemy.
Meanwhile, his lawyers have their careers to worry about, and blatantly lying to congress on national TV, is not going to go over very well with the bar association. So they can’t pursue the defense that is probably best for Trumps own goals. Note that what is best for Trump is not the same as what is best for the Republican party.