Because to Trump, ending the war between Ukraine and Russia means ceding parts of Ukraine to Russia. Russia already took over Crimea with no consequences. If Trump were president, they would have taken over more of Ukraine with no consequences. What is to stop Putin from invading other countries if there are no consequences and he could enlarge his own country with the captured territories?

Trump wants to stop the cartels using the US military which means that we would be invading Mexico, a sovereign country and an ally. Not something that anyone other than Trump wants to do.