It was 2006-2007 when I last ran a series but I had the opposite issue, lots of drivers signed up but they did not enter the first or second round because they wanted to see the series live up to its claims before committing to it, I then got flooded with new applications.
Then as the season wore on drivers who were gunning for glory but who's championships faltered began to drop out.
If I do another it will be a shorter series - about 4-6 races, and I would consider the idea of holding the whole thing in one weekend to maintain as much consistency as possible throughout the event.
In other words, the needs and wants of the drivers is more instant than that of the league. Drivers want on track action at that moment when they want it, but in this era of information overload you are soon lost in the crowd of other past times and events. Where you appealed to a niche interest a week ago, this week you need to appeal to new drivers.
The only tool you have in your armoury is to be as consistent as possible in yourself so that drivers see the benefit of turning up.
By postponing your previous event drivers may wonder what the odds of a race are when they next attend. So in future, if an events attendance is poor I would recommend running it anyway, don't pander to people who havn't shown up - because the most important thing you must do as a league admin is to be consistent.
If you are consistent, then those people who "get the message", which will be about 20% of your signups, will become consistent too.