Because many view anything after fertilization to be a life that must be preserved at all costs. But Graham's abortion bill was for 15 weeks as well.
And they've been looking to overturn Roe since it was decided, and that meant returning it to the states to decide. And yes, I'm sure some would like a federal ban on it, but that's not happening.
Again, JWHO's refusal to accept 15 weeks, which you said it reasonable, is what led to Roe being overturned. You can talk about the Supreme Court doing it, but they unwittingly opened the door to it.