Speaker drivers (the parts that actually move) are mechanical devices which are subject to age and wear.

If the driver cone is supported by a foam or rubberized flex ring that will often stiffen or degrade with time. 'Great' speakers become less great with time. Some more than others, of course.

If there is an electronic crossover circuit inside the speaker, the capacitors can change with time, affecting the sound.

Internal wire connections can oxidize and/or contacts become less tight.