Does anyone know why the spouse of a married participant is automatically the primary beneficiary of 100% of the participants benefit under a plan not subject to QJSA?

I assume this is because the normal form of benefit is a lump sum.

Depending on which type of plan you're referring to, it's not so much that the plan is not subject to QJSA but rather that it meets an exception to QJSA thru certain requirements. Under 401(a)(11)(B)(iii) one of those requirements is that "... the participant’s nonforfeitable accrued benefit ... is payable in full, on the death of the participant, to the participant’s surviving spouse ..." So in the case of a DC plan (other than a money purchase plan), it's by meeting the exception to the QJSA rules that the spouse is the automatic 100% bene.

Because the exception to QJSA requires it.

ERISA, Title I, § 205(b)(1)©(i)

and

IRC § 401(a)(11)(B)(iii)(I)

Thanks a million masteff and J Simmons!