I'll offer something based on my understanding, if I may ?
It's a difficult issue - LEGO are trying to keep a much tighter control of the proliferation of parts (& colors) than they did in the past, to help control their costs, and hence help keep themselves profitable. They recognise that new parts are exciting to their customers, and contribute to sales growth, but also that it is more important to some themes than others. Each year they decide how many new parts they can afford to produce, and allocate a number to each of their teams (& hence to themes). Bionicle typical gets the most number of new parts, as that is seen to be a key characteristic of that theme (I also suspect that bionicle parts are cheaper to produce ...); Star Wars usually does quite well too, as do other comflict-based themes (Mars Mission, Agents, etc.), but the more traditional themes (Town) don't do so well. Each team will propose new pieces, based on a idea, which will be evaluated by management for their potential construction value, before a final decision is made. A new piece's 'value' to other themes will be considered. Its likely that there will often be discussions & 'horse trading' between the different theme teams. I understand they keep prototypes of elements that never made it into production at Billund, for future reference ...
I believe the new cow was designed for the Town/Farm sets, but actually first appeared in the 'Direct' team's Medieval Market Place (the Direct team rarely get any new parts of their own); I think the same is true of the new fish, which was designed for Pirates. So there is cross-over between Teams.
Don't know if that helps :-)