Urban II, whose real name was Odo of Lagery, was from Chatillon-sur-Marne in Champagne, and he first preached the crusade at the Council of Clermont in Auvergne. Since Urban was French, and the council was attended by mostly French bishops, that's basically the reason why the crusaders were mostly French.

It’s also possible that he focused on France because French knights were known for fighting against each other and breaking truces, and this was one way to gather them all together and give them an outlet for their violence. This is probably a bit of an outdated explanation, but it's still true that Urban preached the crusade throughout France in 1095 and 1096. He recruited powerful allies in the south of France such as Adhemar, Bishop of Le Puy, and Raymond, Count of Toulouse. The bishops who participated in the Council of Clermont also preached the crusade in their own dioceses. So, most of the crusaders were French simply because Urban II was French and that’s where he spent the most time preaching the crusade.

As for the Normans, the ones in the Duchy of Normandy in northern France were targets of Urban’s preaching along with the rest of France. The Normans in southern Italy were not specifically targeted, but the Norman kingdom was a neighbour of the Papal States, and were generally allied with papal causes. Urban may have recruited Bohemond of Taranto personally. Unfortunately it’s not totally clear how the Normans in Italy got involved, but once Bohemond joined, a big southern Italian contingent signed up as well.

There were actually numerous crusaders from the Holy Roman Empire, but mostly from the areas close to France. The eventual first king of Jerusalem, Godfrey of Bouillon, was Duke of Lower Lorraine in the HRE, and his brothers and other imperial vassals came with him. However, Urban could certainly not expect help from the majority of the Empire, or at least not from the Emperor and any of the Emperor’s most influential supporters. The 11th-13th centuries were the height of the constant struggle between the Pope and the Emperor for power over the church, and for political power over Germany and northern Italy. The Emperor at the time, Henry IV, was the same emperor who had to walk barefoot through the snow to beg forgiveness from the pope in 1077, so he definitely wouldn’t have been interested in a military expedition that increased the pope’s power and influence.

The first few waves of the crusade did have a lot of participants from the Empire, from as far east as Bavaria and Bohemia. But they were pretty unorganized and disruptive. The Hungarians did participate in a sense, because most of the waves of the crusade passed through Hungary, but since they were so disruptive, the Hungarians were glad to get rid of them. Places like Poland and Hungary didn't send any direct military support because, well, they just weren’t invited! Apparently no one considered asking them. They did participate in later crusades though.

Even though the French and the Normans seemed to be the majority of the participants, people from all over Europe were involved in the First Crusade. According to Fulcher of Chartres,

"There were present Franks, Flemings, Frisians, Gauls, Allobroges, Lotharingians, Alemanni, Bavarians, Normans, English, Scots, Aquitanians, Italians, Dacians, Apulians, Iberians, Bretons, Greeks, and Armenians." (pg. 88)

Some of these are fancy old-fashioned Roman names (the Allobroges are the Swiss, the Dacians could be Hungarians or Romanians…or perhaps Danes), but there were participants from all over the place, even if they weren’t the main participants.

So, the short answer: Urban II was French and focused his preaching on France.

Sources:

Thomas Asbridge, The First Crusade: A New History (Oxford University Press, 2004)

Jonathan Riley-Smith, The First Crusade and the Idea of Crusading (University of Pennsylvania Press, 1986)

Jonathan Riley-Smith, The First Crusaders, 1095-1131 (Cambridge University Press, 1997)

Fulcher of Chartres, A History of the Expedition to Jerusalem, 1095-1127, trans. Francis Rita Ryan, ed. Harold S. Fink (Columbia University Press, 1969)

Penny J. Cole, The Preaching of the Crusades to the Holy Land, 1095-1270 (Medieval Academy of America, 1991)