In those countries where dialect study is undertaken, dialectologists observe that there are today many factors militating against the strict maintenance of older dialect  the standardization of terminology as adopted by national periodicals, news services, radio, and television; the establishment of  prestige  dialects and, through the media, their promulgation; and the huge population shifts that have taken place, particularly in the U.S. since WWII.
The media and population shifts has contributed to variations on the older dialects.