There is pretty significant relation between number of pins and prices of almost every piece of hardware involved. IC package price is almost linearly dependent on number of it's pins and for most mass-manufactured chips packaging is major component of final price. Also wide parallel interfaces are harder to design from interference perspective and to route on PCBs. General shift to essentially serial interfaces for almost everything except main memory is motivated by simple fact that trading silicon area used by serialization/deserialization logic for less device pins, simpler boards and simpler system design is very worthwhile whenever it's at least partially possible. For very slow interfaces it has been this way for at least 30 years, since mid 90's for slow system buses (that's why around 2000 motherboards stopped having ISA slots, ISA was replaced by serial LPC interface around that time) and since early 00's for almost everything in non-specialist applications.