That's because Celtic draw their support from 4 sources. How many millions of supporters?
Eire.
Northern Ireland.
Scotland.
Glasgow.
Compare that to City, where the vast majority of our support comes from Manchester and across England to an extent.
The demographics speak for themselves.
So in theory, Celtic should be wiping the floor with City attendance wise. But that always hasn't been the case.
Then you have to factor in trophy success. Imagine Celtic going 34 years without a trophy. Etc.
And our nearest local rival is probably the biggest and most famous club in the World(I feel sick), bar Real Madrid and Barcelona.
Put all that together and even you must appreciate how well our support and fan base has stood up and stayed loyal over the years.
Even now, along with the 'Emptyhad jibes' started by United, City are still pulling well over 50K for home games. (and with the right prices, another stadium expansion, which doesn't look like it's happening now, City could pull 60K at home, or close to that)