If by “the brother of the Lord” Paul implied that James was the only biological one, as opposed to a fellow follower of Jesus, this would contradict the gospels’ claim that Jesus had multiple biological brothers.

Richard Carrier says, ‘the use of the definite article doesn’t work in Greek exactly like it does in English. It is routine for Paul to say “So-and-so the brother” of many Christians whom even Ehrman agrees are cultic and not biological brothers. See OHJ, p. 589, n. 99.’

That notes reads in part: ‘Some claim Paul’s use of the definite article (‘the brother of the Lord’) is significant, but that is not the case. For example, 1 Cor. 16.12 (‘Apollos the brother’); Phil. 2.25 (‘Epaphroditus the brother’); Rom. 14.10 (‘the brother of you’); 1 Thess. 4.6 (‘the brother of one [of us]’); 1 Cor. 8.13 (‘the brother of me’); 2 Cor. 2.13 (‘Titus the brother of me’); 1 Thess. 3.2 (‘Timothy the brother of us’), etc.’

You ask why Paul wouldn’t call Cephas a brother too. Since “the brother of the Lord” isn’t as exclusive in the Greek, Paul wasn’t implying that Cephas wasn’t a cultic brother too. As Carrier points out, the Greek in Gal.1:19 is quite convoluted, since Paul was tripping over himself to deemphasize the influence of other people on the origin of his teachings. So it’s possible Paul’s convoluted syntax painted him into a corner.