Well, I think there are a few factors.
It's been a good week for Russian players but I think that the simple reason is that they're not mentally tough enough. I don't think Dementieva did anything necessarily wrong against Dokic, just Dokic was a lot more determined to make this tournament count. Elena, however, has improved I think from last year. The problem is, players like Dokic, Clijsters and Henin have improved even further which makes Dementieva look bad, which she isn't.
Before the match, a lot of russian fans said that Elena wouldn't be burnt out and that the crowd would spur her on, after she loses however, there's always an excuse that needs to be found. If you think that Elena was tired, how about Jelena? She's played an enormous amount of tennis and even sprained her ankle during the match but was still able to win.
Dokic, Clijsters and Henin, they all do what Dementieva does except on a larger and more refined scale. They're tough and are determined to win matches. Last year, Elena was beating Jelena and Kim, and this year, it's a role reversal. Jelena should have really won 6-2 6-1 out there as when she served out the sets at 5-2 and 5-2 she had 30-0 leads before she stopped concentrating.
Also, the 3 girls have all had major breakthroughs this year. Clijsters with her titles in Stanford and Leipzig along with her RG final. Henin with her titles in the Gold Coast, Canberra and s-Hertogenbosch, not to mention her Wimby F. Dokic also, with her titles in Rome, Tokyo and Moscow. They're all in the top 10 and work extremely hard.
The problem with the Russian girls is that their fans build them up, to an extent where you expect incredible things from them and they simply aren't good enough to deliver. When they don't deliver, they're called failures etc. I think Elena would probably be happy with where she is at the moment.
Another factor is age, Dementieva is 2 years older than Dokic and Clijsters and they're already better, Dementieva has some catching up to do.