Why Khan but not Matthysse?

[Pu·gi·list]

Why was Mayweather willing to pull up Khan from 140 to a weight Khan had never fought at

He could've done the same with Matthysse right? Matthyssee would've been a better front runner to choose from than Khan. Khan is 2 of 4 in this last 4 fights so it's not like it was anything better than Matthysse