From a Watsonian perspective, maybe the weather was inhospitable in some fashion, or the bike was in need of repairs, or this was before they got a second car, or the second car was in the shop.
From a Doyleist (Wattersonian?) perspective, Watterson probably forgot.
There were a bunch of strips about him being an avid cyclist, so maybe he bike commuted? Of course, maybe they have two cars, as is also common.
But then why would his mum using the car delay him
From a Watsonian perspective, maybe the weather was inhospitable in some fashion, or the bike was in need of repairs, or this was before they got a second car, or the second car was in the shop.
From a Doyleist (Wattersonian?) perspective, Watterson probably forgot.