Wolf Knight is quite right - different themes will do better or worse in different countries and, of course, parents like to have their input and often to meet their play needs! However, when you look at Playmobil over time and overall, certain themes have been long established - knights, construction, police, fire and rescue, pirates and Western. In recent years, Western has become less popular in this group.
There are, however, some themes which have been short lived, notably the aliens, and the ghost pirates are another. In recent years, however, Playmobil's marketing strategy has changed with themes, even popular ones such as the Romans, running for only a few years. So today short life and unpopularity are not necessarily linked.