Probably because of the haphazard way the USCIS processes applications. There are numerous examples of people who applied in the autumn of 2007 and have already received ILs long before folks who applied in July or August. Although the USCIS claims to process applications in the order in which they are received, there is ample evidence to suggest that this rule is not the only guiding principle when it comes to processing applications. One's PD is roughly correlated to processing time, but it's definitely not a sure thing.
BTW, do you know if you are cleared name check? That would be the only official reason for not scheduling your interview. If you are through name check then it is probably just bureaucratic BS. But, it looks like LA DO is making some progress, so hopefully you should get your IL soon.