Forbearers

At these moments	America	has carried on	not simply because of the skill or vision of those in high office	but	because	We the People	have remained	faithful to the ideals of our forbearers*, and true to our founding documents
a					x b cause: reason
Location	Actor	Process: material	Cause: reason			Carrier	Process: relational	Attribute
Adjunct	Subject	Finite/Predicator	Adjunct			Subject	Finite/Predicator	Complement
Theme: marked	Rheme			Theme			Rheme

Reasoning:

The 'but' relates to 'not simply' and anticipates a second prepositional phrase related by paratactic extension in a phrase complex realising a circumstance of Cause: reason.*  However, the writer has instead opted for an enhancing dependent clause of Cause: reason, thereby creating a logical inconsistency.

The hypotactic relation between the clauses can be made more apparent by removing the preceding circumstance of Cause: reason (which also removes the need for 'but'):

At these moments	America	has carried on	because	We the People	have remained	faithful to the ideals of our forbearers, and true to our founding documents
a			x b cause: reason
Location	Actor	Process: material		Carrier	Process: relational	Attribute
Adjunct	Subject	Finite/Predicator		Subject	Finite/Predicator	Complement
Theme: marked	Rheme		Theme		Rheme

* An extending paratactic prepositional phrase complex such as:

not simply because of the skill or vision of those in high office,

but because of the fact that We the People have remained faithful to the ideals of our forbearers, and true to our founding documents 

not simply	because of	the skill or vision [of those in high office]	but	because of	the fact [[that We the People have remained faithful to the ideals of our forbearers, and true to our founding documents]]
1			+ 2
	Process	Range		Process	Range

∞

* forbear

verb: politely or patiently restrain an impulse to do something; refrain.

"he modestly forbears to include his own work"

synonyms: refrain, abstain, desist, keep, restrain oneself, stop oneself, hold back,withhold

Given that the writer screwed up on the lexis, it's hardly surprising they screwed up on the grammar as well, by trying to set up a logico-semantic relation between a prepositional phrase and a clause.

The theory accounts for an instance like this the same way it accounts for instances like he done real good.

On the mismatch between potential and instance, see here.