Unless you are in tight mountainous terrain, or surrounded by high obstacles there is only marginally lower DH/DA going from LNAV to LPV, typically around 50'. And that extra 50' is going to cost you WAAS, which balloons up in cost for your captive OEM FMS systems like you have in the AW139. Also helicopters, especially with newer automation, tolerate the "dreaded" airplane chop and drop profiles pretty well, and in fact provide an advantage for a longer period of time to transition to visual.
And then there is the whole "what lighting do you have at the bottom" question. Standard for an ILS runway, but still in a regulatory quagmire for a helipad.
Why not so many helicopter LPV? Think of it as a return on investment - not a lot of return for the extra investment.