What Excalibre said.
Suffice means more or less “be adequate/enough.” “Suffice it to say,” as mentioned, is equivalent to “Let it suffice to say.” Or, “Be it enough to say” and “Let it be enough to say.”
Grammatically, “sufficed” doesn’t cut it. Something has to be the subject of the verb “suffice.” In the above examples, “it” is the subject. “Sufficed to say” would be like “Was enough to say.” Don’t make no sense. What was enough to say?
One could say “It sufficed to say” (sounds odd because it’s not common, but nevertheless grammatically correct), but that’s in the indicative mood, past tense. “Sufficed to say” is incomplete, by itself it’s meaningless.