I think, in order to be named to the world team outright in Greensboro, C & J would have had to have landed their side-by-side salchows in the short. Everything else being equal, that would likely have given them the win.
You mean breaking the relative tie between C/J and G-G/L in the BOW Tier categories. The other part of this is C-G/L's Worlds experience; their U.S. championship win last year (and they previously placed 3rd and 4th before landing in 4th again this year vs C/J's 5th and 2nd placements in two appearances); C-G/L also were 9th, 2nd and 4th in three 4CCs appearances vs no 4CCs experience by C/J.
And as you reference, C-G/L placed slightly ahead of C/J in this year's sp at U.S. Nats. Not to mention whatever political factors were involved, which would perhaps include consideration of the starting order disadvantage C/J would face at Worlds (not that it couldn't be overcome, but early starting orders can impact judges' scoring).
Apart from BOW guidelines, the selection committee apparently still has the discretion to make any decision they prefer I suppose. And in addition to the scoring comparisons mentioned by Dobre, C-G/L have a higher base value on their elements.
Attending 4CCs will give C/J an opportunity to continue building their resume and their confidence internationally. Then they will have an opportunity to return to their home rink and get a head start on preparing for next season, which they could slam dunk in a big way.
Although we can debate and predict, we can't know event outcomes in advance. That's why we watch.
ETA: I don't envy the selection committee the responsibility they hold to make these tough assignment decisions.
Then why aren't they [C-G/L] on the alternate list for 4CCs?
All we can do is guess. Or you could ask the selection committee and grow old waiting for a response.
I would guess that U.S. fed hopes C-G/L will go home and regroup for Worlds rather than expending any extra energy around being named as alternates to 4CCs, which would be a distraction and a tough turnaround, IMO. Meanwhile, Haven/Brandon are first alternates to 4CCs surely to reward them for their good showing on the GP and their overal BOW, while Lu/Mitrofanov and Serafini/Tran are 2nd and 3rd alternates to 4CCs to reward them for their good results this season.