OK, how about this one. There are just two frail wires keeping you in touch with the rest of the world. Once your modem cord's RJ-11 jack plugs into the wall, your connection can receive interference from the wiring in the building, radio frequency noise from appliances, lightning strikes, whatever. After that, you are at the mercy of the local telephone company to transfer your signal to whatever computer is on the other end of the line.
Modern modems are smart.
They actively monitor the connection and adjust your transmission speed up and down based on how fast they think they can transmit with an acceptable number of errors. When the volume of transmission errors becomes unacceptable, the data rate can bog down to a few bytes per second. Laptop users are often business travelers in hotels with notoriously overloaded telephone trees. In many cases, local telephone service overseas is not as robust as the post-Baby Bell systems in the United States and Canada. A telephone network that was adequate when it was installed in the '80s has necessarily been massively overtaxed by the growing needs of faxes, modems, voice mail, et cetera.