Performance Pledge on network availabilty

The 5 biggest ISPs in HK (PCCW-IMS, HGC, HKBN, NWT and i-cable) have all published their performance pledge on network availability. PCCW-IMS, HGC, HKBN and NWT all pledge to achieve network availabilty of 99.99 %. As for i-cable, it only guarantees 99 % of network availability delivered to subscribers. Actually, 99 % availability means loss of services of 87 hours in a year. I could not understand what makes HFC and cable modem technologies not as reliable as xDSL and Metro-Ethernet.