Google is your friend... drink the kool-aid
FYI, an optical drive is like a CD, DVD, Blu-Ray drive; what I think you're asking is why is it more expensive then a mechanical hd? Right?
In one word: Production. SSD are new tech, which means expensive, new fab processes, machines to make parts, engineering, and so on. Theoretically, once things get going, it should be cheaper to produce an SSD in the future than a mechanical hd. Hard drives have glass or alloy plates coated with a magnetic compound that have to hold a ridiculous tolerance, and an electric spindle motor, magneto-resistive mechanical actuator arms, as well as PCB boards with processor(s) and cache memory. SSD's have the processor(s) and cache memory, but silicone memory chips (NAND) to replace the rest.