Because I can walk through a strict program in my head (or on paper), and see every value, what is being calculated, and what is being stored. With a lazy language, I can't necessarily do that. The entire memory of my program can quickly fill up with thunks. The optimizer determines when things actually get calculated, and I need a much deeper understanding of my code to figure out the space constraints.

Just writing code? I guess I can agree that lazy evaluation makes it easier, but performance still needs to be considered.