If Democrats win the House tonight and not the US Senate while it will be rare I think there are very specific reasons why this will have happened:
1. There were many House seats that flipped from GOP to Democratic in states where the US Senator was already a Democrat and was secure in that seat and hence no opportunity for a pick up. Examples of this are in New York and Florida.
2. There were House seats that flipped from GOP to Democratic in states where the US Senator was already a Republican and was secure in that seat or had no opposition and hence no chance for a Democratic pickup. Examples of this are in Texas, Indiana, and Arizona.
3. GOP Seats in the House and GOP seats in the Senate held solid together. Possibilites here are Montana and Tennessee.
4. The Senate races became about the Democratic candidates while the House races stayed about national issues and the US House support of Bush and all the scandals that mostly affected House candidates. I would argue that Montana and Tennnesse became races about Tester and Ford while many House races were not about the candidates themselves.
5. The states that the Democrats did not pick up in the US Senate were partly due, once again, to ballot measures that the GOP used to pull their vote.
6. The GOP 72 hour plan worked where they were worried months or years ago (Tennesse and Missouri--if they win these) but in the Congressional Districts that they thought were safe and sound in they lost.
Just a layman's view