Much more competitive leisure market
Much more diverse population.
Much more transient population.

Mix those together and you get a lot of challenges to overcome.

The criticism gets leveled at Leeds that they should be getting 25k+ every week because they're in a city of close to 1m people. The problem is that there's a lot more to do in Leeds on a Friday night than there is in most towns in RL land put together and of those 1m people, a lot have relatively little affinity for the area beyond it being a place that they moved to for work or study - and that's what city clubs are competing against.

But historically, when RL was played at 3pm on a Sunday, in a time when shops were closed on a Sunday, then why didn't RL take off in Manchester & Liverpool ?

Why should it? Both have a long and treasured history in soccer (to avoid mixing up the games). I think (no, I firmly believe and avocate) RL is far better than any other type of football. However that opinion is not shared by many. before you point out the query does not apply just to today, I'm approaching retirement and remember what it was like in the 60's: the same.

Let's put the size of population to one side (as it should also include Birmingham) but ask why hasn't it taken off in Wales as there have been periods where the grip of the RU was breakable and they have a history of "egg chasing". I think the issue is that we are a minority sport and suffer from such. Therefore we fade when planted in suitable areas.

That's stretching it a bit you could add a city like Southampton on to Leeds's population if you want to stretch it to a million.

population of leeds is only 450 - 500k but i take your point. the surrounding area brings it up to about 750k but then you are competing with wakey and cas for supporters in the likes of garforth, rothwell, woodlesford et al

Football is the sport in these cities

Its not as simple as "they are football cities", as Leeds, Bradford, Hull and Wigan etc sustain both RL and football, so why don't the larger two cities of Manchester & Liverpool?

Hmmmm.... Bradford City, Wigan Athletic and Hull City compared to Man Utd, Man City, Liverpool and Everton. It's close, it really is
Clearly being 'football cities' is way too simplistic but to pretend that you can compare Wigan to either city in that regard is crazy talk
Mind you it might explain why Rochdale Hornets struggle so much cos they have a football team in their town. probably also explains why Bury Broncos have never grown beyond a community club

Has there ever been stadiums in Manchester and liverpool that are the right size for rugbyleague clubs to play in and grow

The comparison is that whilst Manchester and Liverpool have 2 major football clubs each, smaller towns and cities can sustain both RL and football, so why can't these major cities with a greater overall population?

Historically RL was played on Saturday afternoons. It changed to Sunday's to try to save the game from competition with football.

Manchester does have an RL side - it's called the Red Devils (in a small suburb called Salford).

OK, without being stupid, Salford Red Devils are essentially in greater Manchester with no competitor to fight against in terms of support, have a huge catchment area but us in the Championship get bigger crowds. If you renamed them Manchester, might that help? I doubt it. The problem is both these cities have 2 huge round ball clubs, the other cities don't.