Because the number of very polluting cars entering London has dropped from 35,000 a day to 12,000 a day over the decade since it's come in, the amount of NOx pollution in the City has nearly halved, and we want to do everything we can to get non-compliant cars off the roads ASAP because they put out about 4x as many pollutants as a modern (ie post-2006) car.
The taxpayer shells out £3.7Bn a year for treatment of pollution related illnesses to Londoners every year.
Before this latest expansion, over 75% of residents were in favour. Expanding it is always controversial, but when the 98% of drivers who are unaffected realise they're unaffected and it makes them less like to die from cancer, they'll support it again.