It makes sense for the Beta program actually. Typically in beta you want to keep the scope limited so it's easier to find problems and fix them before adding a lot more content. But if you want people to use the service enough to get good data on it, you need to put up content that you think would most likely bring people in. That would be newer games, rather than old games. The reality is that, in the large scale of things, video game players tend to gravitate much more to current console games rather than the older consoles. In this case since the 'next-gen' consoles are so new that not everyone has moved over yet, this includes the last-gen consoles (ie PS3).
I'm sure once they're out of Beta, or in the near future, they'll begin to add the older console games on there.
Another possibility is they simply don't have the older console emulation working with the streaming service to a good enough degree yet, and it needs more time to get fix up. They would definitely prioritize the newer consoles than the older here for the same reasons above.
Yeah it sucks, but that's just kind of how it goes. Though, really, I'm not banking on streaming games to be anywhere comparable to actually having an offline copy anyway. It's definitely not going to work the same, and I'll likely never jump on board with it.