Just to be clear, Green Lanes towards Finsbury Park is the main bus and shopping route, not to mention bing on the Piccadilly Line. You may have a point about Tottenham, but people always make a connection to the Skolars because they are at opposite ends of the same road, but Arsenal is a local club.
Both are also clubs with long histories and so ingrained in the community, and both play in the most popular sport league in the world. Of course they are going to generate more interest and have a naturally larger catchment than a brand new semi-professional club in a sport that few people in the area have any interest in. Do you really need that explaining?
Precisely, the trains work that way, and that is what people use to travel the sort of journeys that would be required for people in Ham and Eltham to get to their compass point club in Croydon.
The deepest bus routes from Croydon into south east London are to Lewisham or Bromley and both take over an hour while the same takes 45 minutes by train including the wait time at London Bridge or Victoria. Anyone not on the line of those bus routes, which is most of south east London, will find the advantage of the train even quicker than adding extra bus stages.
From south west London there is just the X26 to Heathrow via Kingston but otherwise you cannot get any deeper than Morden, Tooting or Wimbledon by tram. You do not have to go all the way into central London from that side, but you still have to head out to Clapham Junction to change.
The point remains that Croydon is not conveniently accessible to large parts of south London, and whether by bus or train a club an hour away is not going to be considered local or part of a community just because it is all "south of the river, innit". And similar cases can be made for the other compass points.
RugbyLeagueGeek has explained, but domestically to be clear: in American football London has dominated the sport yet most NFL fans do not even know those domestic teams exist. The bulk of the interest in that sport, just like with all others, is in its top professional league.
Even when the London Monarchs played in NFL Europe, and had their derbies with the Scottish Claymores, the attendances were a fraction of those who went to watch American teams come here to play pre-season matches in which they rested their big names. The TV audience for NFL Europe was lower than the NFL because people were less interested in a lower tier league. An aggregate of almost 170,000 people will watch the NFL in London this year, few of whom will ever watch the London Blitz or London Warriors. They will not go to Wembley because they were attracted by local community clubs playing in the park but because they want to see the best athletes playing at the top of their game. Rugby league has no special magical properties that make it any different.
Almost every week on here we see people telling us they are losing interest in watching the likes of Leigh because Championship rugby is not enough for them. So if that is not good enough for those who grew up as fans of the game in a town where it is the only professional team, how on earth can anyone think Londoners, to a number that would grow the game as a spectator sport, would forgo their 6 Premier League clubs, 2 county cricket clubs, 2 premiership rugby union clubs, countless Football League clubs, various other sports and various international sporting events, to instead chose to spend hours on buses (because they are not tourists, apparently) to go and watch lowest tier semi-professional rugby league?
Also consider basketball, it is one of the biggest sports in the world and in the UK it is played in many schools while most parks have hoops these days. The number of kids who grow up learning that sport is a level of grassroots development we could only dream of, yet as a spectator sport it is still almost an afterthought. The two are very different things and need to be developed separately.
I think the thing you are missing is that one-off events will always get a bigger audience than week-in, week-out events. The NFL play one game in London a year, naturally the attendance is higher than for the Monarchs who played every week. And quite a lot of that crowd fly in from continental Europe especially Germany. Broncos would get a decent crowd if they only played one game a year.
Not to mention that American football really isn't a good spectator sport at levels below NFL. NFL Europe was a rejigged version of the World League which flopped badly in North America (since they were unprepared to watch mickey mouse American football). the World League uniquely had much better crowds in its expansion areas than in its heartlands! Quite the opposite of rugby league.
The other issue is that the English invented most sports and have limited interest in playing or following sports deemed foreign. For this reason, there are lots of basketball courts in England but people don't play basketball on them even if basketball is a school sport.
With regard to your point about catchment area. Naturally a higher profile team will have a larger catchment area. Broncos fans do tend to come from all over London. Skolars fans probably don't so much but there are really two kinds of fans. Those who will respond to a new club on their doorstep because it is close and those who are already league fans and will travel. I doubt that all Skolars fans live locally.