Rubinstein only recorded three pieces by Rachmaninoff: his Prelude in C# minor (Op. 3, No. 2), the Second Piano Concerto (Op. 18), and the Rhapsody on a Theme by Paganini (Op. 43). So it's more likely that Rachmaninoff was simply not part of his repertoire; whereas Chopin was well-established as his bread and butter. Rubinstein also recorded more concertos in general than did Horowitz.
Horowitz, on the other hand, recorded a great deal of Chopin, just not his concertos, which are generally seen as secondary works within Chopin's overall piano output. But he recorded a fair bit of Rachmaninoff, with whom he was close friends. The third concerto, in particular, had a special place in Horowitz's repertoire, being the subject of his first recording.
Another possible issue is that Rachmaninoff (b. 1873) and Rubinstein (b. 1887) were rough contemporaries; whereas Rachmaninoff was already a well-established performer and composer by the time Horowitz was born (1903). Rubinstein and Rachmaninoff, to a certain degree, would have been competitors (as performers), Horowitz idolized Rachmaninoff, but Rubinstein's early repertoire would have focused (even more) on composers like Chopin, Brahms, Liszt, and earlier composers like Beethoven.
I don't think it's fair to read much into the pianists' decisions regarding these particular concertos. Factors such as the prior existence of recordings, the timeliness in comparison to other recordings, the opportunities to record, professional contacts and allegiances, and their overall repertorial interests likely played far greater roles than their respective abilities or temperaments.