Hi Olivier,
It's most likely an attempt to have stability... knowing well ahead of time how much money is available for the budget rather than being dependent on the trials of the domain-selling marketplace.
It also frees ISOC from a significant conflict of interest should it want to get involved more deeply in DNS policy.
It may also be a guess/gamble by ISOC that .org has peaked (under the restrictions it had) and this is the time to extract maximum value.