Cisalpine Gaul was geographically contiguous with Italia proper, being on the nearer (cis-) side of the Alps. This was in contrast to Transalpine Gaul, which was the area beyond the Alps. So the first reason is that the Alps formed a natural boundary which divided formerly Celtic lands into two "sides," with the cisalpine side being naturally more within the reach of Roman conquest. It was easier pickings, and much closer and more accessible to Italia than Transalpine Gaul.

Speaking of conquest, that is a second reason. The Roman reach simply did not extend very far into Europe beyond the Alps in the 2nd century BCE and into the first half of the 1st century BCE. Roman interest in western Europe before Caesar's time mostly involved maintaining a land route from Italia to Hispania via Provence, which they governed as Provincia ("Province") and partially anchored via the independent city of Massalia (Marseilles). Rome showed very little interest in or knowledge of the lands beyond the Rhone corridor before Caesar's time.

The two "halves" of Gallia were also culturally distinct. Cisalpine Gaul had been thoroughly "Romanized" by the middle of the 1st century, especially via several large Roman colonies which pre-dated the 2nd Punic War, including Ariminum, Placentia, and Cremona. Later Roman populations were installed at Mutina, Parma, Caesena, Bononia, etc. This effectively "Romanized" or "Italicized" Cisalpine Gaul, such that by the middle of the 1st century BCE and the arrival of Caesar, the two "halves" of Gallia were very much different in culture, language, etc. The Transpadines lived beyond the Po river, but on the Italian side of the Alps, and were virtually indistinguishable from their "Italic" neighbors to the south, except that they did not receive the same political privileges. Caesar used them as sturdy fighters in his Gallic Wars and they eventually received full citizenship.