One thing this reflects is the growing uniformity within parties. Democrats have had the Senate for the entirety of Obama's term, so objectionable bills have hit a previous roadblock. Looking at GWB's vetoes, all but one occurred with a Democratic Senate. Similarly, all of Clinton's vetoes occurred after the Republicans took the Senate in 1995.

Edit: To clarify a bit - The Northeastern secular Republican (Lincoln Chafee, Arlen Specter) and the Southern culturally conservative, pro-social program Democrat (Zell Miller) are both largely casualties of a system where increased awareness in the modern news cycle makes it much easier to paint an individual for the all the actions of their party, leading to a more ideologically pure Senate where all bills that pass reflect the controlling party's ideology.