The reason would be something better playing and not as stale-looking and more expansive coming out soon for likely not much more and also likely to go on sale relatively soon afterward considering
franchise sale patterns.
Basically, it looks interesting, but I have a lot of games to play. If the next one will render this one irrelevant, it's not saving any money even to get it on sale. I would have a window to play it before it gets upgraded in the next game of the franchise, but if I play it out that quickly (fans have had over 2 years to soak it in) then it could have a burnout effect, diminishing the experience that the new game could have been.
Alternatively, if I pace things more slowly so I'm not burned out and wait to pick up the next one at a super cheap price, I may then be in a similar situation again looking forward to the next installment after that one. So then, knowing that the path of spending less now could end up being a case of spending more truly and diminishing overall impact of franchise experience, why would a newcomer get X3 instead of waiting for X Rebirth?