The Gemara says that we don't just ask the owner of the field to bid first on the field but we even force him if he does not want. This is codified by the Rambam, and following the Mishna he distinguishes between when yovel is not noheig (as in our mishna) and when yovel IS noheg, so that we only force this bid when yovel is NOT noheig. Based on the reason mentioned in the Mishna, that of the benefit for Hekdesh, why not force him to make the bid (and thus redeem since there is no bidding) even where there is yovel noheig, so that Hekdesh will benefit the Chomesh? (This is what the Raaavad actually says).
The חק נתן explains that DAVKA when yoveil is not noheig we force the issue. This is because we need to ensure that the field get redeemed, since otherwise it stays kadosh forever, and we have to be chosheish for a takala. However, In the case where yoveil IS noheig, things will get rectified when yoveil comes and it leaves hekdesh anyway.