There may be more to it than that. The advantage of sweep tubes was that
they worked very well at lower voltage. For the same power level, the 6146
required more voltage. Also remember this was before the 6146B arrived.

I am not a collector of Drake equipment (and I am not a fan of sweep tubes),
but didn't the TR-3 arrive first? Three tubes were used for a lot of power.
The use of the sweep tubes there may have set the foundation for later
transmitters and transceivers.

I remember a conversation with a friend some 45 years ago. He was lamenting
that the Hallicrafters HT-32 used the 6146, because it was so wimpy and
actually had lower plate dissipation than the 807 it was supposed to
replace. He returned the HT-32 and bought an old Harvey Wells Bandmaster.
Then worked the world on 10 meters with that rig.

73, Colin K7FM