Traditional hard drives have been with us since the explosion of personal computing in the late 1970’s & early 1980’s. During that time they have got physically smaller, exponentially bigger in terms of data capacity & much, much faster.
These days you can buy 3.5 inch hard drives with 6, 8 or even 10 Terabytes (TB) of space in a 7.2K SATA format. They are starting to fill them with helium which offers less resistance than air, but in essence they are just tinkering with a tried & tested design.
Traditional hard drives are limited mainly by their ability to read or write data to their disk platters. They way they work hasn’t changed dramatically in the last 30 or so years. They have an arm (actuator) which reads & writes data to & from a spinning disk (platter).
The speed that the disk rotates determines the speed at which data can be read or written. These days the most popular (traditional) disks used in Enterprise situations like datacentres will tend to be 7.2K, 10K or 15K (the K represents the number of thousands of rotations per minute, i.e. 7.2K = 7200 rotations per minute).
With SSD there are just no moving parts to have to deal with the data. Data is just sent (written) electronically to the storage space or retrieved (read).
The net result? SSDs are significantly faster & more reliable than their SATA or even SAS predecessors.