I think its because the writers just pitched 'interesting characters' without giving any though to how they would function within the show.
'Oh, the engineer. Okay! Shes part human, part Klingon, and shes like... the fiesty one! Oh! And shes part of the marquis and distrusts the federation!'
In TNG, TOS and DS9 the characters all revolved around who the captain was and what the situation was. They made an interesting team because of how they reacted to each other rather than just existing seperately. Like in TNG, Picard was the humanist, Riker was more of the mans man type officer who could liase between him and the crew and be trusted in tight spots. Data was the naive child who helped Picard reinforce or challenge his ideas on humanity. Every other crew member has a specific skill that they are the best at.
Nothing like that occured in VOY, there isnt any strong character dynamic, and nobody seems to have any speciality. Torres is the engineer, but its seems like anyone else can do her job, and Janeway can pretty much do any job herself.