Theory and practice.
First, it's a question of patriotism, Boeing is one of the pride and sign of American power, USA will not accept that such a prestigious company is acquired by a foreign company.
1st or 2nd civil aircraft manufacturer, one of the first players in military aviation,It is a high-tech key company (the Americans are the only ones with Airbus who know how to build series aircraft, Canada Brazil and China are minors players).
The social concern (153,000 employees - subcontractor - indirect employment), if Boeing coughs the USA will get cold it's the same with GM.
Financial aspect, in a globalized world the avionics market is growing, the company is going through a bad time but it will be able to recover, and a finally a question of sovereignty, key company of the industrial military complex, with Lockeed, Boeing is the entreprise who signs the most contract with the Pentagane and it is unimaginable that USA depend of a foreign company to ensure their national security.
I believe that defence, energy, transport (airport and ports), banks, health are too vital sectors for a country's economy and that a state must intervene when it's necessary despite the mediocrity of executives who didn't know how to run the company.