Its basically the same reason why we really havnt seen CD-ROM speeds go beyond 56x as well.
CD and DVD readers use only 1 laser, and there is a upper speed limit to how fast the drive can spin the disk, typically around 9000rpm, thus there is a limit to how much information the drive can read per second.
There are basically only 2 solutions to higher read speeds, faster spin speeds or multiple laser beams at once.
Obviously spin speeds can only go so high, as vibrations make reading impossible and ultra high spin speeds vastly increases the chance of CD's "blowing up".
Multiple laser drives have been made, namely the kenwood 72x. Unfortunately it had more than its fare share of problems, and never really took off.
There were rumors a year or so ago about a UBER 25x DVD/100x CD drive, but it never appeared in the stores, so one must assume they were never able to get it working reliably.
DVD's can transfer data faster than CD's due to the higher data density and smaller wavelength laser beam, but they are still constrained to the same limitations.
Thus i doubt we will see drives beyond 16x for some time yet.
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