Its as a deterrent for counterfeiting, the machinery used has to be specially calibrated to not tear the more delicate bill, adding another line of defense against counterfeiters. Also its to save money on production
I remember when in the 50's and 60's, US dollars wouldn't tear easy,why did that stop?
It is the same in the UK with 5 pound notes. As the value of the money has reduced, so has the quality of the paper being used, with a high proportion of wood fibre being used as opposed to things like cotton and flax fibres.
