Unfortunately, they've got a terrible marketing problem.
Before 1998 or so, a relational database was an expensive product that you got from a vendor like Oracle. Since then, a generation of people have grown up that think about using a commercial RDBMS the same way most of us think about putting our hands in a toilet.
@va_coder hits the nail right on the head, it's not just the cost of the product, it's the cost of the buying process.
If I want to trial a product that is open source or has an OS or free edition (that could be mysql, mongodb, postgres or even Virtoso OpenLink or SQL Server Express) I can download it, read the docs and play around with it and learn a lot in a few hours. I might learn that the product is not for me, or I might get a positive impression and feel ready to commit coding time to it.
If I want to trial Oracle or Clustrix, well, I'm going to have to start a contact with a sales organization and then they need to pre-qualify me, and then they need to qualify me and then I'll spend a few hours on the phone talking to people (which might take a few weeks in wall-clock time.)
Even if they give me a 30 day free trial, I could easily spend $500+ of my time just getting the trial... And once I've gotten to the point where I'm negotiating with one vendor I'm going to feel a lot of pressure (internally or from my superiors) to talk with some competitive vendors too to make sure I'm making the right decision.
It's a shame because, certainly, a company like Clustrix could use the revenue they get from product sales to support an awesome development team and really deliver a better product. On the other hand, they have marketing channels that are aimed at large organizations that can afford an expensive buying experience... and it's an expensive selling process for them when they've got to do "complex sales" that require approvals from a large number of stakeholders. They've got to pass those costs onto you.
The trouble with this model is that tomorrow's large organizations are today's small organizations. Today, Facebook could afford just about any commercial software that's out there. However, they made critical technology decisions (that are difficult to reverse) back when they were a little company that could only afford MySQL.