Because a disproportionate number of people sentenced to death are non-white.
White 231 (44.9%)
Black 190 (36.9%)
Hispanic 92 (17.9%)
Other 2 (0.4%)The Hispanic portion is difficult to parse, but it certainly seems that African-Americans are very over-represented.
[0] http://en.wikipedia.org/wiki/Demographics_of_Texas#Racial_an...