Medieval literature makes several references to Gascon, Flemish and especially Genoese to denote elite troops wielding Crossbows. However, it isn't exactly hard to shoot a Crossbow accurately and professionalism shouldn't be so hard that you can't train, say, a farmer from York and label them 'Yorkish Crossbowmen'. So what do the labels attached to these soldiers denote exactly? A superior quality over normal Crossbowmen in what attribute?
Not exactly about crossbowmen, but I am currently reading about almogavars, another group of specially trained militia of the age, and I think I can extract some ideas.
Almogavars were Catalano-Aragonese light infantry, with a special ability for throwing darts (some writers of the era claim they could kill knights). Some points for such specialization were:
Training: Maybe throwing darts seems easy, and going to the shooting gallery and pointing towards something with a crossbow is not as difficult as with a longbow. But think that those people, in the middle of the battle, had to:
- chose a good position
- take defensive measures (many crosbowmen plunged their shield in the ground to get some protection while firing/reloading)
- keep a fast rythm, reloading and shoting speedily and accurately
- fix any issue that could happen with the crossbow (if possible)
- get into hand-to-hand when the enemy closes
without becoming panicked. That might not have been not that easy. If those people wanted to go constantly to battle and survive (hint: they were mercenaries) they had to be not "good enough" but "very good"
Having at home lots of old veterans that can train you in that specific form of warfare is not an asset to ignore. Different regions just chose different branches of warfare, due to tradition.
Hiring: If people of your region were famous as crosbowmen, you would be viewed as a more valuable recruit if you followed that tradition. Probably there was no special interest in hiring Genoese archers; you could get a job but probably not as well paid.
Unit formation: Training in the same weapon that your neighbours meant that, went ready, your neighbours and you could join an already stablished Genoese band/company. Similar to modern armies, a unit used to fighting together is way more powerful than just a bunch of soldiers thrown together in the same direction; they would have a higher fighting cohesion, and veterans would have taught the new recruits the fine points of the "art". Apart from that, other advantages of becoming part of a company would have been:
- the individual soldier would be safer from bandits and from being crossed over by their employer
- the employer had not to manage the individual soldiers. He only dealt with the leaders of said companies (imagine composing a new unit from raw recruits and having to manage 20 Genoese, 14 Venetian, 14 Swiss, 20 Spanish crosbowmen in the same unit). Instead, they get an already trained, prêt-a-porter band of crossbowmen.
For example, in the almogavars example, the hiring party contacted the captain of the force (Roger de Flor), negotiated a salary for each soldier category. The captains hired and directed the force, all the customer1 had to do was paying2.
NOTE: It is worth noting that the description of almogavars do not reduce them to infantry, counting light cavalry between them. I do not know to what extent a unit of Genoese crossbowmen would have had "auxiliary" forces.
NOTE 2: One of the mentions form Ramon Muntaner's Crònica mentions the importance of having good crossbowmen in galleys. Given that Genoa was a merchant city, that would have been a reason for having a constant demand of crossbowmen (but then, that is only one mention by one author).
1: Aragonese kings and the Bizantine Emperor.
2: Other similar mercenary forces mentioned in my reading seem to work similarly, each having a captain of the same nationality of the soldiers.
It's not a question of "accurate shooting", it's a question of becoming expert in the use and maintenance of a crossbow and connection to an industrial culture for making and repairing them. Real crossbows are quite sophisticated devices and in their time were very expensive.
The mercenary corps of crossbowmen were relatively wealthy soldiers who invested time and money developing themselves as elite soldiers with elite equipment.
Your question is kind of like, why is Seal Team Six so elite? All you have to do to fire a machine gun is pull the trigger. There is more to being a soldier than just standing in a field and pulling a trigger.
Crossbows were "slowfiring" weapons. In this regard, they were less valuable than longbows.
The advantage of a crossbow was its accuracy--in skilled hands. In this regard, it didn't take much to be a "random" crossbowman, but it did take a lot to be able to fire a few arrows accurately. That's not too different from a "marksman" or a "sharpshooter" being more valuable than an average "rifleman." That was especially true in the days of slowfiring rifles, before the introduction of mass-fire "repeating" weapons.
During the Hundred Years' war, there were "legends" of French crossbowmen shooting 30, 40 or 50 or more Englishmen, one by one, in battle after battle, firing only two or three shots for every casualty inflicted. That was a MUCH better ratio than longbowmen, or "random" crossbowmen enjoyed. Likewise, at the battle of Stalingrad in 1942, there were individual Soviet "snipers" who were credited with killing "hundreds" of Germans, firing only two or three shots per casualty inflicted.