...would that be because Heathrow is so badly connected to the rest of the country?
this is with the direct ~2h trains (which is slightly faster than you suggest) you want. The model showed that on an HS2 Heathrow train as many people boarded a train to central London (staying on to there - far more got off in West London) as boarded a flight.
Now total Heathrow traffic was worked out as about 3-5%, not the ~1% that 10% of 2tph suggests - OOC was found to be acceptable for North to Heathrow traffic with well over half of it using that connection. As Heathrow trains either mean lower frequency to London from Manchester and Leeds, or the loss of somewhere like Stoke from the HS2 network, the change at OOC is acceptable, and the vast majority of people on Heathrow trains would prefer OOC and Euston, there's no reason to serve the branch unless you want to make the net user experience worse!
From Manchester there would be 3tph (as opposed to 1 direct and 2 with change at OOC) to OOC where a change would get you to Heathrow in 2h20.
I should also point out that terminal 2 would require changing trains whatever, with the connecting service not much different OOC or Heathrow HS2.
But this is going way off the original topic!