It's worth noting that Emirates fly London-Dubai 8 times per day. That's an extraordinarily high number of flights / seats when so many people are flying to Dubai just to connect to somewhere else.
NY is more of a destination in its own right - many more people on that route want to go to NY rather than connect onto another destination. It should also be noted that London-NY links 2 of the world's largest commercial and financial centres - the number of bankers, lawyers and directors of large companies with hefty expense accounts is quite extraordinary