Why can’t deductive arguments have only two supports or idea boxes?

Three supports are needed to form a deductive argument

A deductive argument is based on a chain of deductive reasoning in which two ideas – a statement, and a ‘however' comment which qualifies that statement – lead you to a third, which is the conclusion or implication.

If you only had two parts to the deductive chain, they would necessarily be a statement and a conclusion (or recommendation), leaving out the qualifying comment, which is a pivotal part of the deductive chain.