Why do so few cars have curtain airbags as standard?

eg only a couple of Vauxhall Corsa (2006 on) have curtain airbags as an option or as standard. These versions don't even have it as an option!:-

• Expression
• Life
• Breeze
• Active
• Energy
• Breeze plus
• Active plus
• S
• Exclusiv
• SE
• Design in Touch
• Sxi in Touch
• SRi

Only some of the versions of these Corsa models have curtain or side airbags as standard eg:-

• Club 
• Sxi 
• Design

Search Autotrader for Vauxhall Corsa and 10766 cars are listed. Of these 153 are fitted with curtain airbags just 1.4%!

The Good News is that from 1st January 2010 face-lift most Vauxhall Corsa (SE, Exclusiv, SRi, SXi) models have curtain and side airbags as standard. The entry level S, Expression & Energy models can have curtain airbags fitted as a £425 Protection Pack option. (front seat side-impact airbags, full-size curtain airbags, Pyrotechnic front lap belt pretensioner system).