There's several reasons, though economy would be the main one, as any color would add expense, so it's either white on black or black on white.
It's white on black because it's more pleasing to movie theater viewers who've been sitting in darkened room to read the text white on black than have the jarring opposite of a blinding white screen with black text. In silent movies, intertitles were mostly black with white text, again as it would be jarring to have it in reverse. Also, with the earliest silent intertitles, one could conceivably save time/money by filming a title card written in black type on a white card and then not have to reverse the negative image before cutting into the film. So an perhaps an industry standard was born. Later silent titles were hand drawn white on black, often with borders or other flourishes.