The short answer is that many investment firms issue tax documents in mid-to-late February. By waiting until around March 1st, we ensure that the most recent investment information is captured in your tax documents, including dividend payouts and any company-specific events like mergers and acquisitions. While this doesn't guarantee that you won't receive a restated tax form, it minimizes this chance.
This change, which many firms have adopted, is the result of a law change and IRS report from 2009 which noted that because a significant number of investors were receiving restated forms, the deadline for tax documents should be pushed into February. You can read more on the IRS website here.