Its a cultural thing re London and Union.
The sport is very much seen as a posh lads day out, and offering dirt cheap tickets to see Wembley provides the perfect excuse for them to get drunk.
Rugby League on the other hand is steeped in inter club and town rivalry. Its a parochial passionate family sport, so putting on a similar even just does not work amongst Leagues fanbase.
Something similar to league can also be seen with soccer matches. When Arsenal played European games at Wembley their best attendance was 73,707, which is significantly smaller than what Union got today. However i doubt any sport fan would go around saying Saracens are the biggest club side of any sport in the South East.
The figures look good for someone not familiar with the sport. However the reality is that in a week or two when they move back to Watford they will be back to their usual 6000.
The time to get really worried about club RU would be if they were pulling in huge crowds on a week by week basis. The reality is they are not, they are broadly similar to those in RL. This is despite the far higher media profile they have.
That in itself should be a real worry for RU.
The simple fact is that the biggest threat to RL is from the very insecurity and inferiority complex of many of its fans.