There are likely many reasons SpaceX did not include something of scientific value (they have not yet responded to a request for comment). One would be continued management of the mission after the launch; as it is now, the Tesla has simply been left to wander through space alone.
Another is the oft-cited Musk quip that the rocket had a decent chance of exploding. He repeatedly said that other rocket test flights included only "concrete blocks" as dummy weights. Why not have more fun and send a car?
That’s not strictly true, though, as a number of test flights have included useful payloads. The Ariane 5 in 1996, for example, included four ESA spacecraft known as Cluster (although the rocket exploded on its way to orbit). The Atlas V’s first flight in 2002, meanwhile, launched a group of satellites called Hot Bird 6.
The Falcon Heavy’s main competitor, the Delta IV Heavy (half as powerful and four times more expensive, mind), did fly a dummy spacecraft as a payload on its first launch in 2004 – but it also carried a few useful satellites built by students, too. SpaceX even included something useful on its first Falcon 9 launch in 2010 – a prototype Dragon spacecraft.
Wallace, for his part, said he had a shoebox-sized deep space navigation system that could have ridden shotgun with Starman on Falcon Heavy. Plenty of others, too, would no doubt have jumped at the opportunity – even with the “50-50” risk of explosion.
Falcon Heavy is facing dwindling demand; Musk said they are already putting more focus on its successor, the Big F*cking Rocket. Its main market may well be scientific missions, which does make this seem like a bit of a missed opportunity
Of course, there is another side to the argument. It is (mostly) Musk’s money, and it is Musk’s company. True, they’ve had considerable funding from the US government, and they are leasing launch pads from NASA. But why shouldn’t he do what he wants?
The launch of the car was, without a doubt, fun. I’ve had plenty of non-space friends talk to me about the launch, who otherwise wouldn’t have been interested. And, to boot, it was the biggest launch in more than a generation. That deserves plenty of praise.
And there's little doubt about the significance of the launch. Falcon Heavy is now the most powerful rocket in operation today, and the biggest to launch (in terms of payload it can lift) since the Saturn V's final launch in 1973.
Still, it might have been nice to see something else on the rocket too, alongside the car. And yes, everyone has already made the same joke.