I would be tempted to say it is because of tax consequences, but I not an accountant.
I think it is more likely that if they do the pension first, it is because the Ratty and Stinky thieves know how to get to the pension $. For a 100 years, the T pension was the most over funded pension in the US - mostly because of all the people that dropped dead before they could collect a dime.
Then back in '98 T figured out that they could rob the pension by paying retiree health benefits out of the pension rather than out of operating expenses thanks to a Klinton era Executive Order - another theft enabler.
Now the T pension is so grossly underfunded that I bet they will have to borrow more $ to pay out all the lump sum benefits that we will be demanded if they were to freeze the pension and everyone MR75 eligible were to leave like I plan. Maybe that would cause it to go bust entirely.
On the other hand, I doubt T has a way to rob our 401K. That is completely separate. However, I am sure the Kongress IDI0TS have plans on robbing our 401k all drawn up!