Prop 187 was, in part, a deliberate attempt to force a rehearing of Plyler v. Doe. Appellate courts are loathe to overturn Supreme Court rulings. Any decision on Plyler was only going to be resolved in the Supreme Court. The authors of the initiative should have known this.
There was no severability clause in Proposition 187. Without a severability clause, all that the opponents needed to do was get ONE very small piece of the initiative overturned, and the entire thing went with it. And it was 99% certain that some parts of the initiative were going to get shot down. The guys who wrote the initiative didn't think ahead, and the thing died right there.
Yeah, it was badly written. The opponents used that to get the whole thing thrown out. Tough s**t.