I honestly don't know. At a guess, it's because he was dead-set against consolidating the BI process from the disparate departments under OPM.
Which made sense - used to be departments had their own clearances, and they didn't map. Bush standardized them on DoD definitions across the government, then moved the background investigation processes to OPM. But it was hugely unpopular with the departments themselves (because who wants to give up power and appropriations), and with some in Congress, of which I think Cummings may have been one.
But while that's an educated guess...it's still a guess.