Why aren't there multiple testnets? It seems like a strange idea to only be able to run one experiment at a time. I can appreciate it still requires miners and nodes but I would've expected they'd allocate resources to different concepts.
Good question. I assume it's mostly due to lack of interest. In order to be able to test things on a network replicating bitcoin's one this network would need to have a decent number of participants. Starting an entirely new testnet out of the blue to test something temporarily likely wouldn't attract a decent amount of participants willing to put effort and resources into building it and maintaining for the tests. So the existing test net was used.