Why hasn't Congresswoman Betty McCollum yet cosponsored H.R. 676 Medicare for all single payer act?

I just discovered that Democratic Party congresswoman Betty McCollum has not yet signed on as a co-sponsor of  H.R. 676 Medicare for All bill that has already garnered 112 cosponsors.  Here is the official listing of the co-sponsors.  I thought about this after watching a Jimmy Dore youtube video where he interviews a guy who is primarying against Nancy Pelosi -- Pelosi also not yet a cosponsor and has said words to the effect that she is against single payer.  This makes me wonder if maybe I should primary against McCollum, even though she doesn't represent my residential district.  That would be a little ironic because Represents congressman Jason Lewis 'represents' my district, but resides in McCollum's district.

Democratic constituents, in these congressional districts that lean so heavily Democratic that the Dem candidate is all but guaranteed to get elected, should see the primaries as brass ring opportunities to get the most progressive candidate possible.  By that I mean there isn’t much of a need in those districts for a candidate to compromise in order to acquire the moderate fence sitters that might get put off by the far left progressive positions of a candidate.  But instead, it seems more so that the opposite has occurred, whereby the constituents of perennial House Democrats settle for progressive mediocrity in electing very conventional neoliberal, corporate friendly candidates.