Manchester has effectively been in tier 2 since the end of July so has had to deal with restrictions for longer than Merseyside and Lancashire. I believe that the levels of poverty are also higher.
Why do people keep saying this
Most of Lancashire and half of West Yorkahire have also been under the same restrictions as GM and for the same length of time
There are other places in the north just as deprived, if not more so. Bradford for example, a larger city than Manchester (the city), has had consistently high cases since March and been under the same restrictions as GM and Blackburn (Lancashire) too.
Inspite of what Burnham says, GM is not an exceptional special case in the north