The logic for a Khmer font is already profoundly complex.
This would require creation of a large number of combination glyphs: [3 pre-base vowels] * [2 variants of COENG RO] * [35 consonants] * [3 ligature forms/each] = 630 glyphs
Apple is already having problems securing significant adoption of its font technology. The struggles font developers have to go through to get a working font for complex scripts is further discouraging that development.
Application developers are in turn discouraged from incorporating ATSUI support when they know that few intelligent fonts will be available to take advantage of it.
In the past Macintosh surpassed Microsoft Windows in its internationalisation support...and although it still has an advantage that a script can be added by independent developers without the OS publisher having to be cajoled into supporting it, the difficulty experienced in adding complex scripts to the Macintosh negates its usefulness in the real world. When is Apple going to get back its vision for being the computer for the rest of us?!