I agree there needs to be more software for BBs, but here's why there's so much more for Treo:
1. Palm platform has been around for ages (around a decade, I think), and at one time they had something like 75% of the handheld market. Lots of developers learned to work on Palm, and then kept with it. Blackberries were only moderately hot a few years ago, and have only gotten REALLY hot in the last two.
2. The Blackberry platform is more restrictive. I have dabbled in *DKs for Palm OS, Windows, and BB. The BB platform is by far the most complicated for making a quality application. Because you don't have a touch screen, it actually requires some serious and somewhat unconventional thought from the developers about how they're going to design the interface for their application. As an interface designer, I think this is a spectacular thing, because too many programmers don't know jack-**** about interface design. The downside, of course, is that we don't have as many apps.
3. The last big reason is that the majority of BlackBerries are corporate devices and many people aren't allowed to install other software on their devices. And any software that IS developed is usually done so for a specific company, or by the IT department of your own company (as mine does), and it never sees the light of day beyond your own employees.
But I too have a desire for more (free!) applications. We can only hope that since BBs have recently become so very popular we will soon see a surge in application development.