Here is the FAR 25 reference regarding max alt and 8000' cabin.
Most transport category jets are capable of flying higher, however the limit is the 8000' cabin requirement . A higher pressure differential to mainmtain the 8000' at a higher altitude would require a higher pressure differential which would require a stronger/heavier sructure. AT 39,000' a diff of about 8.6 PSI is required.
http://edocket.access.gpo.gov/cfr_20...4cfr25.841.pdf