The wiki says the first testnet reset was done because testnet coins were becoming valuable. Isn't this kind of stupid? To have a testnet that has any worth at all, we NEED it to be a mini version of Bitcoin. This means we need to replicate Bitcoin's community in a miniature way. This, in turn, means testnet coins NEED to have value, or there's no point in having a testnet.
I'm pretty sure that the Bitcoin developers don't want to be responsible for their own coins competing with each other. Testnet would start to be viewed as "new and improved" Bitcoin, and that's not what they are trying to do.