Heathrow probably has enough passengers to spread the cost of pre-clearance so the extra per passenger isn't too much. However, financially their only reason to do that would be to increase passenger numbers, but they don't have the capacity to carry them.
For Manchester those economies of scale are much less, making the economics that much more challenging. It has more population in its surface transport catchment than Dublin, but lack's Dublin's comprehensive feeder flight network, and residents of Ireland probably have more propensity than Brits to travel to the USA due to historic family ties. CdG or Schipol have some of the same advantages but probably lower trip rates to the USA and for the British market they have the disadvantages of doubleback and EU entry/exit requirements rather than CTA.