Why are cab units preferred for passenger service?

[Triplex]

I think that it is more of a weight issue.

Cab Units are lighter because they are built with an open lattice or Monocoque frame.

Never thought of that.

However, there are plenty of examples of Hood-Units in Passenger Service. HEP is in the short hood.

Many early passenger hoods had the steam generator in the short hood (requiring it to be high), but most later S-G and HEP installations are in the rear of the long hood.

Passenger hood units I can think of offhand:
"torpedo tube" GP9s (SG in nose)
some Trainmasters (SG in nose)
"hammerhead" RS-3s (SG in high short hood; RS-3s were long hood forward by default)
"Tempo" RS-18s (HEP in high nose)
I know a number of other first-generation types came with SGs, but I often don't know which because it didn't change the model number.
GP30Bs (SG in short hood)
some Mexican GP38-2s and GP40-2s (SG in high short hood)
GP40P, GP40P-2 (SG in rear of long hood)
GP40TC (HEP in rear of long hood)
several types of GP40/-2 rebuilds on commuter lines and Alaska RR (HEP in rear of long hood usually)
SDP35, SDP40, SDP45 (SG in rear of long hood)
some Alaska RR SD70MACs (HEP in rear of long hood)
P32-8BWH (HEP in long hood?)
U28CG, U36CG, some Mexican C30-7s (SG behind cab)
U34CH (HEP in long hood?)
Long Island C420s (SG in high nose)
some Mexican C628s (SG behind cab?)