There may have been some Scottish, Hungarian, and Polish crusaders, but for the most part, those countries were not directly targeted by crusade preaching. So they didn’t join in huge numbers mostly because nobody really asked them to.
Pope Urban II was French (his real name was Odo of Lagery), and the Council of Clermont was mostly attended by French bishops. At first, apparently he only intended French soldiers to go on crusade. Why only the French? This is one of the oldest and biggest questions among crusade historians. He may have focused on France because French knights were known for fighting against each other and breaking truces, and there may have been a lot of landless knights with nothing else to do but wander around and fight each other. This explanation is a bit outdated now, but it was one of the reasons given by medieval chroniclers (who were, not coincidentally, priests and monks who were often attacked by knights).
Urban himself 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. The Normans in Sicily and Southern Italy also joined in large numbers, as they were neighbours of the Papal States and were allied with the pope.
In the Holy Roman Empire, crusaders mostly from the areas close to France, such as Godfrey of Bouillon, the duke of Lower Lorraine. But Urban and the Emperor were enemies, so the pope didn’t ask for his help. Still, news spread quickly, even if there were no official preachers there. There were some crusaders from Bavaria and Bohemia.
For the three countries you asked about, Hungary is the easiest to deal with. A later medieval legend suggests that King Ladislaus I of Hungary was actually chosen to lead the crusade. But that can’t be true, because Ladislaus died in July 1095, months before the Council of Clermont. In reality it seems that no one ever asked the Hungarians. They were certainly involved in the crusade though - the crusaders all agreed to meet at Constantinople at the end of 1096, and the ones who went overland walked through Hungary. The Hungarians tried to escort them towards Byzantine territory, but violence occasionally broke out, so they were mostly glad to get rid of them.
The Hungarians were also busy dealing with other problems on their borders, such as the Cumans and Pechenegs. But the Hungarians did participate in the later crusades. King Andrew II was one of the leaders of the Fifth Crusade.
The Polish were also simply never invited. They did have a lot of contact with France and the Empire, so it’s likely that they knew about it. They were also already fighting against the non-Christians who lived further north, which they felt was a holy war of their own, similar to the crusade. Later, a Polish army fought in the Second Crusade, and there were other Polish expeditions and pilgrimages to Jerusalem in the 12th century. There’s also a funny story from much later in the 13th century, when Duke Leszek the White said he couldn’t go on crusade because they didn’t have any beer or mead there.
The Scots are a bit more complicated. Two French chroniclers of the crusade mention seeing Scottish crusaders. Fulcher of Chartres notee the people he saw and languages he heard from all over Europe:
“And whoever heard such a mixture of languages in one army? There were present Franks, Flemings, Frisians, Gauls, Allobroges, Lotharingians, Alemanni, Bavarians, Normans, English, Scots, Aquitanians, Italians, Dacians, Apulians, Iberians, Bretons, Greeks, and Armenians.” (Fulcher of Chartres, pg. 88)
Some of these are intentionally archaic names - the Allobroges were an ancient Gaulish tribe who lived in the Alps, so Fulcher means contemporary Swiss. “Dacians” could mean people from ancient Dacia, maybe referring to medieval Bulgaria, but it could also refer to Denmark. And this is the problem with his mention of “Scots” - the Scots who lived in Scotland in the Middle Ages originally came from Ireland, so Fulcher might be talking about Irish people instead.
Guibert of Nogent talks about all the crusaders who participated in the Siege of Antioch:
“Although the call from the apostolic see was directed only to the French nation, as though it were special, what nation under Christian law did not send forth throngs to that place? In the belief that they owed the same allegiance to God as did the French, they strove strenuously, to the full extent of their powers, to share the danger with the Franks. There you would have seen the military formations of Scots, savage in their own country, but elsewhere unwarlike, their knees bare, with their shaggy cloaks, provisions hanging from their shoulders, having slipped out of their boggy borders, offering as aid and testimony to their faith and loyalty, their arms, numerically ridiculous in comparison with ours.” (Guibert of Nogent, pg. 29)
This is also a bit ambiguous, since he could be referring to the Irish as well. And Guibert wasn't actually present on the crusade at all, he was relying on descriptions from other people.
So, the short answer is that Pope Urban II was French and focused his preaching on France. The Poles and Hungarians were far away and they were never directly invited, although the Hungarians were involved because the crusade passed through their territory. There is more evidence for Scottish crusaders, but they may be Irish, and in any case there weren’t very many of them.
Sources:
Thomas Asbridge, The First Crusade: A New History (Oxford University Press, 2004)
Jonathan Riley-Smith, The First Crusaders, 1095-1131 (Cambridge University Press, 1997)
Penny J. Cole, The Preaching of the Crusades to the Holy Land, 1095-1270 (Medieval Academy of America, 1991)
Mikolaj Gladysz, The Forgotten Crusaders: Poland and the Crusader Movement in the Twelfth and Thirteenth Centuries (Brill, 2012)
Denis Casey, “Irish involvement in the First and Second Crusades?” in Crusades, vol. 13 (2014)
Christopher Tyerman, England and the Crusades, 1095-1588 (University of Chicago Press, 1988)
Alan Macquarrie, Scotland and the Crusades, 1095-1560 (Edinburgh University Press, 1985)
Fulcher of Chartres, A History of the Expedition to Jerusalem, 1095-1127, trans. Francis Rita Ryan, ed. Harold S. Fink (Columbia University Press, 1969)
Guibert of Nogent, The Deeds of God Through the Franks, trans. Robert Levine (Echo Library, 1998)
I'm not aware of any specific works about Hungary and the First Crusade, at least not in English, but Asbridge's history talks about the crusade's passage through Hungarian territory.