por h o por b

Presumably, as a general excuse when learning (Spanish) orthography, since the use of the letter B is mostly etymological, and not intuitive at all (bearing in mind that, in most Spanish-speaking regions, the sound of b and v is exactly the same), and the use of h is even more arbitrary, since the letter h is completely silent and its use is pure convention.

It is also common to see it written as "por hache(s) o por be(s)" ("hache" is the Spanish pronunciation of the letter h, and "be" of b), both singular or plural.

por h o por b

1. (idiomatic) for one reason or another