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Is there any reason why MUL operator is only in single operand form?
IMUL operator can be in three different forms (with one, two or three operands) and this is much more convenient. From the technical point of view I don’t see any reason why MUL operator can’t be in two/three operands form.
It has to do with the bytecodes that are output. In the pre-80286 world there were too many opcodes so the Intel engineers were finding ways to overcome the problem. One solution was to extend the portion of the bytecode that specifies the operation (multiplication in this case) into the portion of the bytecode that encoded the first operand. This obviously meant that only one operand could by supported when executing the MUL opcode. Because a multiplication requires two operands, they solved the problem by hard coding into the processor that the first operand would always be the eax register. Later processors supported bytecodes that were of multiple lengths wich allowed them to encode more data into a single command. This allowed them to make the IMUL opcode much more useful.
There is an interesting parallel today with the IP addresses running out.