Because, I told you way back in March what happened. Haeckel's drawings faded from prominence as comparative embryology faded. He never disappeared completely, but non-Haeckelian topics and drawings certainly dominated things (contrary to the way you think things were, with Haeckel being everywhere).
In the 80's, comparative embryology reappeared. Since it hadn't really been a part of evolutionary study or focus since World War I ended except in the most minor way, the field basically had to start in the modern era at an early-20th-century level. And, since Haeckel's original drawings were the last comprehensive and purportedly scientific data set about the subject (as opposed to small drawings in textbooks that just fine for getting general concepts across but were useless for actual scientific study) to really have been produced, Haeckel's drawings were dusted off and re-examined.