It's tough to know for sure. The Wachowskis are notoriously difficult to both interview and, even when they do interview, get straight answers from.
Joel Silver was the producer of the film and, among many other things, acted as their mouthpiece for most of the interviews involving the film. In a discussion with Indie London the following exchange took place:
Questioner. What was the appeal of Speed Racer?
Joel Silver: The idea was to
make something really exciting and fun for the whole family. The
Wachowski brothers wanted to make a big family movie and I think they
did it. They wanted a movie that everyone can enjoy – you wouldn’t
just take your kids to it and drop them off and pick them up. It was
always the intention to create a great experience you can have
As for why the Wachowski's don't talk to the press:
Questioner: Why aren’t the Wachowski brothers doing any press for this?
Silver: The Matrix was the first movie we did together and they did
everything – junkets, tours and everything. But they didn’t like it.
They didn’t like explaining the movie and telling what they were
thinking; they wanted the movie to take the lead with their art and
their creation. They didn’t want to have to present what they did. So,
they said to me: “Listen, if you really want us to make these movies
with you, we don’t want to have to do this anymore. So, it’s very
simple – if you do that for us, then we’ll continue to make movies
with you. If you don’t, we won’t.” But we love them and they are
around. They’ll be at the premiere. But they just don’t like to talk
about it and we’re happy to support them in that. If you’re hinting at
the mystique surrounding them, they don’t cultivate that, they just
want to let the movie speak for itself.
I don't know if that's the answer you were hoping for, but it really does seem like they just wanted to do something big, fun, family-related and different.