In that time period, it made perfect sense. You talk bout 25 cent boxes as if they were commonplace. For 90% of readers, if they got to one convention a year they were lucky. In 1975, there were two comic shops on Long Island, as I recall. Back issues were hard to find and the demand for these books was strong. At least half the books I bought were reprints.
Marvel owned the product, all they needed was a new cover and someone to edit the stories a bit. Marvel Tales, Double Feature, Super-Action, Marvels Greatest Comics, and a half dozen more titles were solid enough and took up valuable shelf space. Marvel/Cadence made pennies per book sold so volume was key.
Not to mention the irony of Marvel publishing more books each month featuring Kirby than they did when he actually worked there.
The X-Men had a bit of a cult following and had recently been guest stars in a few books, but economics said if might be more profitable to keep it in reprints.