The answer is for various reasons.
Sevilla aren't skint but operate on a tight budget. There's some rule in La Liga this summer that clubs have to "balance the books" in terms of transfer deals - i.e. they can't spend more than they bring in*. Lopetegui wants to bring in certain players and therefore has to sell. The sale of Gil was more or less forced on him, BG didn't want to sign a new contract, with a much higher buy-out clause, so he didn't really have any alternative
Lopetegui likes playing with inverted wingers and overlapping full backs, so I'm guessing Gil was either told or he worked out that his game time was going to be limited (certainly on the left). We did very well to come up with a deal that suits all four parties and get in before the others that were sniffing about.
*I'm not sure about the details of this but Marca keep referring to it. I'll try to find out how it works, whether wages are factored in and so on