There are a lot of reasons why the Rugby Union World Cup drew significant larger crowds than what the League one is drawing.
Public Perception of the Product:
The 2000 RLWC caused a situation where the Rugby League World Cup was seen as a 'tarnished' brand, verses Unions World Cup where in 2003, it was riding on a lot of goodwill from the success of the Wallabies in 99 and the large international program Union had running at the time.
Advertising Budgets:
Rugby Union had millions to spend on advertising, League barely had thousands to spend.
Sponsorship:
Union has the strength of being able to sign partners like Coke, Reebok, Visa and so on for long deals that cover multiple World Cups which help give further cash to underwrite and push a tournament. By League not having one for 8 years, there was not a sponsor family for international league to draw upon, and thus it was the NRL's sponsorship family that had to come to the party.
Ticket Prices:
Especially for low key matches in the Union World Cup you could purchase a significant portion of tickets for between $5-$7. There were also plenty of giveaways and so on. For whatever reason in League, a low key match will still cost you between $15 - $35, which is a fair bit more expensive and for a tarnished, less promoted product, does make for a harder sell... on the plus side it has meant a fair bit of revenue still for the RLWC group
We have to be realistic with the crowds, and so far the crowds aren't too bad. Honestly, outside of say Soccer and Union, what other sport would draw larger crowds for its World Cup tournament?
Cricket maybe...though recently for games not involving Australia they have been struggling and also the 2003 and 2007 CWC's averaged 11,193 and 13,176, figures we are currently ahead of.