Why havent the Libs insisted on Proportional Representation ? their main issue?

Constituency electoions are an unfair method, because they never reflect the total vote nation wide The Libs and to some extent Labour have been the victims of that. Why didthe Libs oin the Tories and gave up on an issue which would have one them many more MP's Even Labour in order to get the Libs into oalition agreed to introduce PR. This is very bizarre behaviour coming from the Libs and I wonder what the real reason for that is ?