It is all in the type of dye used in the manufacturing process and any past/current/future environmental influences (UV exposure, temp  etc.).  

http://en.wikipedia.org/wiki/CD-R

Commercially produced optical discs have the data physically stamped into them (similar to a record, but read with a laser instead of a needle).

Consumer writers on the other hand use discs with dyes or phase-change materials which change in reflectivity when 'burned'. These dyes are more subject to breakdown due to light, temperature, etc.

Yup. Expose them to UV light and they'll degrade in a hurry. 

Any media will degrade over time. I have albums, cassettes and VHS tapes older than a lot of members here. When played, I play copies and only use the originals as masters.  That said,how they are handled, stored and the equipment they are played back on is a major factor in their lifespan.  External drives go bad as well. Your best bet is multiple backups/copies.

Yeah, I know. Simply don't have the means to do that.

What Kryo said...