The low mile marker is 194. Perhaps I-17 and I-19 were going to be the same road at first. This is an intrastate Interstate.
After writing that, I got the following email from Richard C Moeur
I believe (with very strong corroborative evidence) the reason that I-17 begins at MP 194 is that I-17 follows the route of old SR 69 for about 60 miles. Where SR 69 and US 89 used to meet (at the current intersection of I-17 and Grand Ave), their mileposts (MP 201.63) would equate. Arizona, up until a few years ago, generally had a policy of not starting mileposts over at the beginning of new routes, but instead continuing the mileposting upward from the same number. I-17 then inherited the old SR 69 mileposts as it gradually became a freeway. There never were plans for I-19 to extend north of Tucson, but since I-19 and what was US 89 originate near the same point (Nogales Border Inspection Station), it wasn't a bad theory.