"Primitive" is the waste basket category used by ARS-GRIN to indicate that an accession appeared to the evaluator to show little indication of intentional agricultural improvement. I suspect that means simply that those varieties did not seem to resemble any of the high-yielding, prettier commercially developed varieties. Many "primitives" are shorter, have smaller leaves and lower leaf count. They may or may not have higher than typical nicotine levels.
They certainly should not be dismissed as somehow less worthwhile than non-primitives. Some possess traits (e.g. disease resistance) that had been bred out of commercial strains. Many appear to be Orinoco types, and should be interesting for various uses.
Another twist is that the class assignment may be simply incorrect. As an example, ARS-GRIN has 4 accessions (at least) of Guácharo. One is misspelled (Guarcharo) and classed as primitive. One is classed as flue-cured, and two are classed as cigar filler.
So, I would take the judgmental term of "primitive" with a grain of salt.
Bob