Indeed.
To what benefit? It's a clear market split because hub and spoke is prioritised at LHR and beach leisure is left at Gatwick. There's no business reason to keep it there if you get more space at your main base. Why duplicate staff and engineering costs when the market will move to LHR with you? What commercial benefit would there be in duplicating a split operation? None.
The commercial benefit is that routes such as Montego Bay, Male etc. are point-to-point flights taking Brits on long-haul holidays. They do not require hub capacity therefore will never be served at Heathrow.
If BA had the space to increase traffic at Heathrow, they would look for new markets, such as those in South America and other major cities and hubs across the globe to offer direct flights to Heathrow and increase frequency on existing markets to maximise the amount of connecting passengers through its Heathrow hub. They would also need both tourist generating and tourist receiving services like those to major cities and hubs to do this.
They will never waste capacity at their hub by filling up slots with long haul flights to the Caribbean taking Brits on long-haul holidays in the sun for 2 weeks and routes that don't require hub capacity!
That's what Gatwick is for and why the markets are split this way. And as davidjohnson6 points out, the passengers these routes are attracting will not want or need to travel to Heathrow when they don't require the potential extra costs and time or a hub network for their travels.
What's the commercial benefit of becoming too dependent on one base by moving leisure routes into the hub when a bulk of your passengers don't even want or need a hub feed of traffic?