Doubt if it's too avoid LHR as all domestic routes are at LHR-5 and connecting is dead easy if staying on BA and relatively easy otherwise.
No it's probably more to do with:
(1) only 7 UK airports offer flights to/from LHR; and
(2) pre-clearance at DUB avoids the aggravation at the US destination (rather than at LHR).
Aer Lingus connections are popular because,
Price, service offering, faster connection times, customer service and the ease of transfer and excellent OTP.
Pre clearance does not impact a lot as many don't know about it from Europe and the US side.
You comment about domestic UK flights to LHR being low, Edinburgh is the top UK connection point and it has over 10 flights to LHR per day. Bristol is close to LHR yet its second in line for connections. Turning to Europe Amsterdam has endless connections all over the world, yet its the top connection airport for EI to the US.
Now people are booking single tickets to DUB and then onto the US with EI and even connecting further from the hubs with JetBlue and United.
KLM or BA don't offer any advantage only higher prices to keep airport operators happy, longer flights, longer connection and poor service. Time for them to up there game.