Of course the US space program thought of using pencils. The problem with pencils was that the graphite pencil “leads” are quite brittle. On Earth, that’s not a problem; you can dust the chipped bits of grey stuff off your desk and carry on with your day. But in a low gravity environment, graphite splinters and particles float through the air, damaging delicate equipment and (cringe with me) getting into people’s eyes. Health and safety was the motivation behind the pressurized pens. And the Russians adopted them, too!