The metaphysics of A-T is focused on defining an ontology of reality, but it doesn’t do so well at ontogeny. It describes how things are, not as much about how things got the way they are. This may be why origins is harder for A-T.
I have a theory though. Aristotle was an eternalist. Because he didn’t believe the universe had a beginning, he didn’t need to think about ontogeny or origins in the same way as us.
So, when we think about Thomism, to what extent is it unduly influenced by eternalist?
Can you first clarify what you mean that origins is harder for A-T? Are there any specific examples in mind? (Are you thinking of the A-T insistence on defining humans as rational souls, creating possible issues with models of Adam and Eve?)
Terminology quibble: it’s true that Aristotle believed the universe did not have a beginning, but “eternalism” usually denotes to the position that past, present, and future times all exist in the same way, in contrast to “presentism” which holds that only the present exists. Aristotle was a presentist in this sense.
I don’t know of a specific name for the position that the universe had no beginning.