Well there are at least 2 simple reasons.
The normal hardisk is as the name implies a magnetic disc that spins mechanically that's why they often have the RPM on the sales description as indicator of their read throughput.
The harddisk is usually hanging on the south-bridge of the motherboard which is built for slower rates.
That's why CPU, RAM and the graphic-card are usually on the Northbride. North and south are in this case like a map -> up and down on the motherboard.
With RAM there are also two different standard models we have the S-RAM and the D-RAM. The S-RAM is far superior but it needs more parts so the industry went with D-RAM in general.
The D-RAM needs one capacitor and transistor per data bit.
The direct access on the CPU caches L1, L2, L3 are usually built on a S-RAM architecture because the needed size is significant smaller.
Back to the question, RAM are built with transistors and capacitor. a capacitor reacts on the electric field which is built on electric suspense.
Since nothing with a mass bigger than a particle of light can get to lightspeed (e=mc²). And an electric field-wave is equal to a light-wave and electric suspense is based on an electric field, the reaction time of a RAM is within nanoseconds.
The hardisk however has a spinning disk and a reading head. it has to cross a minimum physical distance to get the data it wants.
That's why it's slower, it usually react within milliseconds.
However there are more modern versions of hard disks without moving parts. These are called solid-state -> because they are solid inside. They are built with chips and they react on a nano second scale.
Does this answer help a little?