Because subsequent decisions upholding Roe have upheld it with a slim, 5-4 margin and the 5th vote was Sandra Day O'Connor's. Rehnquist, Scalia, Thomas and Kennedy have voted numerous times to overturn Roe. Do you think that somehow Scalia, Thomas and Kennedy are going to suddenly have a change of heart, now that O'Connor is gone? Bush will be appointing a new Chief Justice and replacing O'Connor. It's naive to think that Pro-Life groups aren't lining up lawsuits as we speak. They would be foolish not to.
By the way, the Supreme Court reverses past Supreme Court decisions on a regular basis. You commonly only ever hear about huge ones, like Brown v. Board of Education, which reversed a bunch of Supreme Court Decisions that said segregation was okay (not to mention Supreme Court decisions that said Slavery was okay). Other, less controversial decisions decision overrullings happen pretty regularly. And, like I said, if it comes to a vote Thomas, Kennedy and Scalia are going to vote the same way on the issue that they've always voted. They're just likely to be on the winning side now.