Your question applies to all battlefields, not just Vicksburg.

Simple answer to a simple question: Northern regimental associations had more money to spend. Along those lines, don't you imagine the State of Illinois spent more than a few dollars on the Illinois memorial? This webpage gives a cost of $194,423. 92 in 1906. Anyone care to estimate what the cost would be today?
http://www.flickr.com/photos/jddean/5521405645/

The State of Alabama is justly proud of her memorial erected in 1951. It cost taxpayers $150,000 at that time, but honestly, is there any comparison?

http://www.civilwaralbum.com/vicksburg/illinois.htm