Is the attendance really that pitiful?
IIRC out of 19 possible cup semi-finals in SL, Leeds have been in 15 and Saints 14.
A cup semi final isn't a special event anymore because we're in them so often. If we lose this one the likelihood is we'll be in one next year.
Whilst there are things that could be done to help boost the attendances, what would really help is if other clubs would pull their fingers out and put up a fight in the earlier rounds and so knock out some of the big 4 clubs before the semi finals.
What the cup needs is 4 or 5 years of Leeds, Saints & Wigan not making the semi's. Then I think you'll see an increase in crowds.
Not to mention that whilst cup crowds are lower than the past, the league attendances are significantly higher. So partly a societal issue too in that more and more people are season ticket holders.