John Doe wrote:
waiting to be launched. So why not put a few smaller modules in the cargo
bay instead of a spacehab ?

I can think of several reasons:

- Weight and CG

there may not be enough weight and CG margin to bring two modules,
while there is for space hab. Spacehab with supplies could be less
dense than, say, a truss segment.

- Crew voted for bringing food & supplies for themselves over bringing
hardware for the station.

- There aren't two modules that will fit in the cargo bay together.