This situation is usually blamed on a combination of poorer defensive and offensive prospects along with their positions in the center of the map. They can readily be attacked from multiple fronts. One of the main reasons for the persistence of this opinion is the poor record they compiled in the early years of tournament play. The truth is that every Great Power has about an equal chance of winning. The playing style demanded of each is different and some are not as easy to master as others.
Turkey is probably the easiest country to play and Italy is perhaps the most difficult.
Your initial playings of Diplomacy will likely be solitary outings against AI-run Great Powers, an excellent way to master the intricacies of the game and test your theories on tactical evolutions and strategic ploys. It also introduces the player to the art of negotiation, albeit on a somewhat simpler level than dealings with a human player. In general, your negotiations with the AI will be more straightforward than with a human opponent. This is diplomatic negotiation stripped to its basics. The alliance and attack orders, along with the hearsay option, deal with broad strategies, aimed at the stance toward a Great Power rather than being concerned with tactical options.