I think it's a combination of a reflection of our times, and people who just don't know any better.
On the one hand, I think that there are many, many drivers out there who frankly don't care about funeral processions. They want to get somewhere, there's something holding them up, and they're not happy about it. Chalk it up to the "me" generation, the fast-food generation, the increase in road-rage or whatever, but there are a lot of very angry, impatient people out there driving. I had one of them behind me the other day, who was very upset that the lane we were in wasn't moving when he could clearly see that the intersection was blocked and there was nowhere to go. He slammed his truck into reverse, hastily maneovered out of the land and into the next where he sped forward exactly 2 car lengths. And you could see him still yelling and agitated. Someone like that, they don't care about funeral processions - all they care about is getting themselves to where they're going.
I also think that a great number of people don't know how to recognize a funeral procession, or don't know what to do regarding a funeral procession. Daytime running lights are law here, so all the cars have them. I don't recall seeing little flags or anything like that for processional cars, but I think that would be a darn good idea. I think they should also do some radio or tv reminders occasionally - one of our stations does them with regular driving tips so maybe they can add funeral processions.
But I think that in general people think that someone dying just has nothing to do with them so why should it affect their day... why should someone else's funeral stop them from driving where they want to.