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.
You can't or rather shouldn't compare the crew of the Enterprise: the Flag ship of the Federation with the Voyager. Only the top graduates of the Academy serve on the Enterprise, anyone else serves on all the rest. Sorry but it's like comparing the crew of the Queen Mary to the crew of a Carnival Cruise ship. Picard is an ambassador to the Federation. That's the title you take upon being captain of the Enterprise. That's why Riker didn't want to captain any other ship. LaForge graduated top honors and is blind, Torres dropped out. How is there even a comparison? Torres can't even serve on another starship after Voyager because she doesn't even have a degree.
The point was, Voyager was a ship of average people with an extraordinary task set before them.
Regarding VOY, I'm not sure how much behind-the-scenes, office politics played a part in who got how much screentime and development. It certainly didn't hurt Jeri Ryan to be in a relationship with Brannon Braga during her time on the show, to be honest. Though I thought Ryan is a good actress and Seven was an interesting character until they decided to make the show mostly about her.
Did Robert Picardo have to suck cock too for his part to increase?
No, it's simple economics.
Paramount spent millions of dollars in advertising to promote the character to the general public. So if you tuned in to see her, there she was because her image sold TONS of merchandise. Otherwise, she'd be a wasted investment. Paramount was giving the audience just what they were paying into. Does Tuvok even have a T-Shirt with just him on it?