The only reason I could think of right now is that Lucho wanted 2 left footed CBs from the beginning because we already have 2 right footed with Pique and Bartra.
But the whole Marquinhos saga would make absolutely no sense then. I don't know. Pretty hard to understand this situation.
IMO, it does make sense if you look at it from a different angle. The difference between a long term solution and a short term solution is that a long term solution needs to be:
1) a WC CB,
2) whose strengths maximally complement our weaknesses (i.e. the best fit possible).
TS is that kind of player which is why we pursued him for so long, when that failed we turned to Marcos who is not WC yet but has that potential, and like TS is a great fit for us. When that failed, we turned to M/V; players who are not WC but who are an ideal fit for us (i.e. have pace, aerial ability, are left footed). The reason being that Lucho and/or the board felt that they could no longer find a CB who was available and matched our 2 criteria, so when a long term solution was no longer possible they opted for a short term one.
While Benatia is certainly better in terms of quality than any of the aforementioned players (except TS), he is also worse in terms of fit than any of them. So buying him would have made him a less than ideal long term solution, which is perhaps why Lucho and/or the board decided to pass, but certainly not because of his price. After all, the board was willing to buy Marcos + another CB = 50-60M, enough to even meet Roma's 61 asking price.