Sorry OP!!!!
I think it could be a combination of all the things that people have mentioned so far.
Usually, it is the older family members who are the 'glue' and if only one is sure to be attending... and other relatives are kind of holding off... nobody is sure who else will be there, etc...
While it certainly is not 'fair', the drive could also be a factor. Now, I have driven 2-4 hours MANY times to attend family functions. I know that I am the odd-man-out, as only one sister lives farther from 'home' than I do. And, I am well aware of the demands that this places on the majority if they are the ones expected to travel. If there are families with children, then the traveling and overnight accommodations, food, etc.. can be quite demanding.
Also, people are just SO busy... there are the big sports events, camps, etc... that the kids are involved in. Work functions. Family functions on the OTHER (inlaws) sides of the families, Precious family vacation time... Which is just now a matter of days with how the schools operate. Etc... Etc... Etc....
However, with all the above considered... this may be the year where the family reunion may not be the primary priority for many.
If this event is an 'outdoor' barbecue/picnic in the heat of July, I know that I would not, personally, be excited to drive that far for what is planned.
I do think that perhaps the hosts have not really taken a serious grasp of how to make something like this 'inviting' and 'exciting'... worth traveling for...
If I were you, and I wanted to maintain the annual family reunion, then I WOULD go ahead and make your plans.
Let everyone know that you will be there, and you hope that other family members will also be there 'with bells on!'.
I think that at the last minute, family members who are able to fit this in may attend. There may be more than you would think!!!! 