One reason might be their simple incompetence. Trump hires lawyers who fawn all over them and they're often not very good as a result. Seasoned sharks in suits want nothing to do with him because he's notorious for not paying his bills, but he desperately needs a lawyer who knows what he or she is doing.
Another reason might be that they're simply trying to whip up Trunk's shrinking base into pressuring Congress to stop Mueller. Not likely that will happen. While a few Trump lackeys on Capitol Hill are still carrying water for him, their numbers have dwindled.
The real reason, however, may be that Trump's lawyers have now realized the transition emails will reveal Trump and/or people close to him, including possibly his sons and daughter, as well as his son-in-law, lied to the FBI and other federal officials.
As Martha Stewart inmate would reluctantly tell you, lying to the feds is a sure-fire way to land yourself in prison. If Trump's children or son-in-law are charged, I suspect Trump would attempt to pardon them before they reached trial, which likely would trigger a constitutional crisis.