Probably because you seem so pissed off with both Otamendi & the club for having the audacity to do the deal.
The frustration is in the lack of faith being shown to Mangala, what kind of message does this send to him?
He's hitting form, looking settled after a difficult year, looking to be forming a solid partnership with Vinnie - and now we want to exchange him for the latest 'must have' centre half?
Look at that from his perspective, it must be baffling.
Otamendi is a lad he played alongside at Porto, both were available in 2014, we pipped for Mangala at more than 30 mil, and Valencia took in Otamendi for £8m.
Now after Mangala had one difficult year, and Otamendi a good one, we're more or less making an admission we signed the wrong player.
How can that not be puzzling?
That's not to even mention that there's no guarantee Otamendi would settle in immediately. Could you imagine if he had a year like Mangala's last season adapting here, and Mangala continued his recent form in Spain and had a breakthrough season?
We'd look foolish in the extreme, and we'd be weaker for it. There'd be no guarantee we wouldn't have upset Mangala as well, and he wouldn't look for a move.
I see the benefits to signing Otamendi, as others have pointed out, but I can't escape the negatives to it, potential and otherwise, they're absolutely glarling.