Schedules are dynamic and can change from month to month and week to week (many routes also have fewer flights on slower travel days like Tue/Wed/Sat). There are actually 5x daily flights currently on both MCO-JFK and MCO-LGA. These can operate anywhere from 5x daily to 7x daily from now until May. You are just not seeing these because you are searching for MCO-LHR instead of MCO-JFK/LGA. DL2503 is MCO-LGA and I would not recommend that flight to connect to a JFK-LHR flight. The fact that DL2019 2PM MCO-JFK does not operate every day is not at all strange and calling DL will not cause them to restore that flight on the day that you want. There are actually many possible connecting options, but delta.com search will not show them all. ITA Matrix can be used to show many of the options not showing on delta.com. I limited the layover time in this search, but if you want to actually spend some time in NYC, you can change the search to show longer layovers (up to 24 hours is legal on international TATL connections).