Number one it's a choice.. Manufacturers by now all being concentrated in ROC,
follow the contract , and the parts manager for the importer company makes those choices.

More silly Blanket GENERALIZATIONS, but it wes no doubt to stir controversy
an discussions amongst people who have nothing to do with the design or economic decisions
of a bike manufacturing corporations , but

I'll play. in a Contrarian Curmudgeon role (as usual)

not all do.. nobody does anything all the time, the same
without being a single purpose Machine, assembly line serf,
Or under threat of sacking from your Job..


2 Examples..

Trek had one with a shimano Roller brake on the rear hub, disc brake on the front ..

Sun Spider, a fat bike, has a coaster brake kick back 2 speed, with the fork
& front hub set-up to take a disc brake, (just not included to keep the Selling price down)

[my old SAAB had disc on the front, drums on the rear.

(drum is a simpler mechanisim to use combined, as a parking brake.
hold a Hot disc, stationary, as a parking brake can cause it to warp )



even side pulls , Campag uses a double pivot front, single pivot rear in their top range,

[Both dual pivot on the lower range component sets.. whole different economics there ..