In this recent question Why is the double bass the only instrument in the violin family tuned in fourths?, there was this answer:
The early history (from about 1500 to 1850) of bass tunings are very variable, with anything from 3 to 6 strings and tunings in anything from thirds to fifths. Reference.
In the classical period, the virtuoso Domenico Dragonetti played a 3-string bass tuned C G D an octave below the cello, which was more or less the standard instrument that Haydn, Mozart and Beethoven wrote for.
The current "standard" (E A D G tuning with a string extension down to low C) is a relatively modern invention.
This answer has 6 upvotes and 1 downvote, but it doesn't address the question. The question is clearly about Why the bass is tuned in fourths. But this answer says this kind of tuning is relatively new, and earlier in history there were other tunings as well.
I flagged this post, but it was declined
declined - I don't think the questions presupposition of an explicit reason makes this answer less useful.
I do think that it is useful as a comment, but as an answer to the question per se, isn't it a bit useless?