Jack -- Because you're confusing a virtual world as a platform and a virtual world as a product.
Consider AOL back in the day. It wanted to be a platform -- the place where people would go for shopping, news, everything. Like of like Microsoft Windows became a platform, and everything happened within that proprietary ecosystem.
Or the way the Apple App Store is a platform for mobile app developers, and all the other device makers got together on Android because you can't have too many competing platforms. You can have one winner, a couple of folks duking it out for second place, and everyone else either dies or survives by finding a deep niche and hanging on tooth and claw.
Second Life wanted to be a platform. Just like AOL wanted to be a platform. But, after the rise of the World Wide Web, it turned out that AOL wasn't a platform after all, but just another product. Think of how many websites there are out there now! Imagine if someone came along early on and said, "Hey, Yahoo! and Google and Amazon -- you can't compete against AOL. Each of you is just delivering one piece of the puzzle. Join together and you might have something!"
Now, with the hypergrid, it turns out that the same is true of virtual worlds. A virtual world that is connected via the hypergrid isn't a platform, but just one destination among many. It's the hypergrid that's the platform, and the more different virtual worlds there are on it, the more attractive the hypergrid is as a whole.
The World Wide Web wouldn't be particularly attractive if all you had was one website you could go to. It's the variety, the constant turnover and innovation, that makes it compelling and attractive.
Same is true for the hypergrid. All those tiny little worlds, each doing something different, with a different business model, different audience -- they are enriching the hypergrid.
But what about virtual worlds that aren't on the hypergrid?
Personally, I think that's just a matter of time. Many of the biggest commercial closed virtual worlds have already announced plans to turn on hypergrid connectivity. Some are waiting to finish up other projects (like Kitely), others are waiting for hypergrid security permissions to get all the bugs beaten out of them (Avination), and many of the rest are just waiting for the big guys to make the switch, and then they'll follow along.
Some grids will always stay closed, just as there are proprietary games that you can't access via the Internet -- even games that require proprietary hardware! They provide a higher-end experience to users, can innovate faster, and want to protect their intellectual property. School and company grids might also stay closed, for security reasons (such as they do now with their intranets), while keeping a region or two up on the hypergrid for marketing and outreach reasons.
But, overall, I believe the variety of options on the hypergrid is a good thing, and will continue to get better. Eventually, even Second Life might join in. After all, AOL did, finally, get on the Web.