That's what I've been wondering for a while.
I've seen other euro-prospects falling in his situation. While it's really an obvious move from the standpoint of the signing team, because you don't want to see the talent you heavily invested to develop just leave to the US, other players I had seen falling into this were either:
a) Developing talents who weren't thought to be able to land soon in the NBA
b) Players in tough financial situations
c) Players with weak advisers who went for the $$ without second thoughts
(b) is unlikely to be the case as I've never heard he comes from a tough background.
The reason is probably a mixture of (a) and (c). Of course, he went for the $$ you'd say, as he is currently payed 160,000€ but that was an amount that Unicaja unilaterally raised until recently. Before that he was making 80,000 (half). And the hype surrounding him was already there at 16 (he had legendary performances in the U-16 euro tournament).
Still, rumors have that the 6 million Euro buyout for the 6 year deal he signed were right there from the beginning, independent from the salary raise (which was probably made to make Rubio happy and make look the 6 million buyout reasonable).
Which I find odd. He signed a 6 year deal when he was 16, with a 6 million buyout. He always have had his family as his advisers, and either if every one was in for the money, or if they were aiming at the NBA draft, you'd say a 6 million buyout isn't reasonable for a 80,000 € contract, nor signing a 6 year deal at 16, when there were NBA possibilities just 2~4 years ahead.