Considering how bad the VFX of Babylon 5 look compared to - dare I say it - DS9, JMS really went for the truly cheap.
Like he had any choice in the matter:
Trek budget - approx. 1.4 million per episode.
B5 budget - approx. 900K per episode.
It's simple math. Don't think for a moment that B5 would have been renewed if it had gone over budget.