The H107D video Tx and camera are all one piece. I've only seen people take the 107C HD board (if they get lucky and find one w/ video out.. there are like 3 versions) and camera and use that to replace H107D camera/board. In addition to this, they also need to add a vTx for FPV. So to have HD FPV in 107D you need the right 107C HD board/camera and add on an additional vTx. Sounds like a waste, some extra weight, and not very expandable going forward.
My reasons are...
1) As you mentioned, better camera... but with less parts (107C HD video board, camera, and vTx vs camera and vTx)
2) Longer and stronger video transmission range
3) More flexible antenna options
4) Adjustable vTx mw -- battery life vs range
5) Possibly shave a few grams of weight
6) Do a first build for experience w/ cheap parts that are already on hand and don't need to purchase
7) Ability to swap out just the FC board for another Cleanflight brushed option as a phase 2 FPV build.. or even make the whole FPV portion modular to go from frame to frame
8) Maybe remove FPV from this setup at some point and run just a Cleanflight board for crazy acro and tune-ability
9) Why not?
It just gives me sooo many options going forward if you look at it like a phased learning process/build log.