Because it still contained citizenship, which he opposed. Cruz supported legalization then, and later, despite denying he ever supported it during the debate.
Schumer said that without a path to citizenship, there is no bill. Then Cruz introduced an amendment removing citizenship from the bill. Then he was one of the leaders of the fight against it.
And somehow, all this means that he was in favor of amnesty and that the amendment (removing the one thing its sponsors said there could be no bill without) was NOT a poison pill.
Sure, whatever the Establishment says.