I've never understood that either.
I'm not quite sure either but I believe some groups ask not to have their own translations used by other groups to translate to another language (i.e. not to go from japanese to german to chinese to english and so on) so the specific language nuances are not lost... well that's just how I understand it (￣∇￣")
Groups tend to abandon a project if someone else was doing it first because of respect so that the other group's effort doesn't go to waste (like this case)
Groups often don't allow retranslation because they often are the ones who buy the raws, if a new group wants to translate they should at least support the artist by buying their own raws.
If its retranslation of a retranslation, its usually cause the permission is not theirs to give, so whoever wants to retranslate should ask permission of the group who owns the raws.
There's plenty of other reasons why a group might not allow it tho.
Off topic but I'm a little a lot of confused on the whole translation thing. If one group is translating another group is not allowed to? I think it's one thing if somebody reposted somebody elsewhere work as their own but another if they are doing the work themselves. Even using the basis of the work of they are translating to another language i think it's fine because they are putting effort into it. For example a group translates from Spanish to English but tells another they cannot use their translation to change it from English to say German or something...it doesn't make sense to me since each group would have to put in their own work...so why not since either is technically illegal anyway (though I humbly appreciate the work of all scanlators and translators)