People from Ireland migrated to many parts of the world and seem to have substantially populated many of the English speaking countries; so perhaps that is why they appeal to so many people from so many different countries. Even people who wouldn't normally read poetry will read limericks so what is it about them that appeals to people.
I think it could be because a good limerick is largely nonsense. The more nonsensical it is, the wider the appeal to our sense of humour. With a well phrased and constructed limerick the pictures that our imaginations form can be as varied as the people listening to the limerick. Words have different colour and emotional overtones for anyone listening.
The beauty of limericks is because they are considered to be a nonsensical form of poetry and therefore not supposed to mean anything serious; means that anyone who is a clever wit can construct a sharp satiric message caged under the auspices of supposedly fun meaning.
This can be a quite devastating form of attack because it's virtually word "guerrilla-warfare" and anyone who responds to something they consider as an attack on them or their philosophies will be seen as a loser. Satire has been used as a form of verbal ammunition since time immemorial and a well constructed, humorous message fires the ammunition with all the force of a Howitzer gun at its intended victim.
Considering the origins and history of the limerick, it most likely came into existence as a passive weapon of attack and denigration. This would have occurred during the time of the Irish uprising and rebellion. Ever since that time of anger and skirmishes, the limerick has proven to be an effective weapon over a long period of time for anyone witty and devious enough to construct a memorable satiric ditty today and appears to have no loss of admirers in the twenty-first century.
It is interesting to note that many ballads have been formulated around limericks. Some of these ballads have been immortalized over a century and are now called folk songs and are sung in many countries outside of where they originated from and so much of the history behind the original limerick has been lost. Today's historians can trace the origins of these folk songs back to limericks set to ballads sung in Ireland.
When so many Irish people emigrated from Ireland during the great Potato famine and went to the colonies, they took these limercik-ballads with them and today are still sung in folk clubs around the world.
Limericks are a fun game to play with words when teaching E-S-L [http://e-s-l.org] as a second language to non-English speaking people.