It’s a question of when Oswald would have read the newspaper
If it is not until Thursday, then the reason is it’s risky to take a rifle in a package into Dealey plaza on Friday morning,let alone into some other building
On the other hand, if Oswald read the Tuesday paper then he had more than enough time to explore Dealey plaza, select some spot or building, and to carry a package with rifle. If William Castor could walk around carrying TWO rifles as well as EXHIBIT them without any DPD , FBI, or other security being alarmed, then surely Oswald could have probably done so with just ONE package