The study looked at new defendants sent to death row, not just the number of executions, so delays in cases because of the Baze lethal injection controversy don't play into the analysis.
Doc Berman said "I assume these remarkable numbers reflect the tendency of prosecutors seeking few death sentences and juries handing out fewer death sentences," and I think both those things are true. The Quintero verdict shows juries are responsible for at least part of that trend, which may also be influencing prosecutor decisions.
I suggested another reason for the decline in the comments:
there's a third cause for the declining number of Texas death sentences: New standards in the wake of the 2001 Texas Fair Defense Act to ensure capital defendants have adequate trial counsel. Better lawyers on the front end means fewer death penalty results on the backside, whereas in Harris County, in particular, the quality of counsel in such cases was historically, notoriously low before that law passed.
As evidence of this trend, more than 60 counties in West Texas recently banded together to jointly fund a capital public defender program, solving the dual problem of 1) the large cost of DP defense and 2) the lack of qualified counsel in many rural counties. So some of this may represent shifting public attitudes, but it may also demonstrate what happens when capital defendants receive effective, zealous legal representation compared to a more lackadaisical variety.
Another commenter noted, "The real reason is that Texas recently added life without the possibility of parole as a sentencing option." An anonymous lawyer suggested more reasons:
Additionally, a relatively new method of jury selection, the Colorado method, helps counsel get -- where they are adult enough to know they won't win the guilt phase -- life verdicts much, much, more often. Finally, a change in culture in the defense bar where we call a "win" in a capital case not a NG / "not guilty" but any verdict other than death encourages lawyers to seek life at all costs.
I've not heard much about institution of the "Colorado method" of jury selection as part of this debate and hope to learn more about that later.
Finally, yet another insightful commenter suggested what's likely the biggest reason for Harris County's dip in death row contributions:
For Harris County anyway, political turmoil is probably the largest reason no one was sentenced to death in 2008. Former DA Chuck Rosenthal resigned in February 2008 and an interim DA was appointed until an election could be held in November. I don't have the numbers, but I wonder how many actual death-penalty trials were held in 2008 in Harris County, rather than being delayed so that a new, elected DA could make the necessary decisions.That's probably true. There's a good chance interim DA Ken Magdison chose to slow down these high profile cases in deference to the office's past dysfunction and its new incoming leader.
When a major change occurs in public policy outcomes - like a county sending up to 15 defendants per year to death row then none just a few years later - often there's no one reason but instead it results from the conjunction of many trends. That's what appears to have happened in Harris County, where I think all of these described trends contributed to this year's surprising goose egg for new death sentences and Texas' overall trend toward decline.