Over at the Conspiracy, Eugene Volokh discusses why an issue might be decided under the federal constitution rather than a State's constitution.
The answer for this in Virginia would be easy: because there is no enforcement available for any right under the Virginia constitution. These rights are "co-extensive" with federal rights. In other words only federal rights will be enforced.
However, rumor has it that other States actually give meaning to the rights in their constitutions (go figure). EV offers a couple reasons why he thinks the Missouri Supreme Court decided the issue based upon the federal constitution rather than its own.
As a practical matter I'd offer one more: the availability of case law. Quite often State law may have only one or two cases concerning the issue you are arguing - if the issue is arcane enough it may have none. This can be maddening when you are trying to brief or appeal an issue. The solution? Use case law from other jurisdictions. Naturally, this approach is not going to find cases which directly address your issue under your State's constitution. However, with the great number of State appellate courts and federal circuits churning out decision after decision based on the federal constitution there will almost always be a couple courts which have opined upon your issue. I think this causes rights based arguments to almost always lead with the federal constitutional argument. There's just so much more to work with when addressing things federally.