Halogens can be analyzed by ICP-OES as well. But of course all ICPs cannot do this. Just google with key word "analysis of chlorine by icp". In the results you will get an application note from Horiba scientific and you can down load the PDF. This will clear your doubt. Now regarding why is it difficult to analyze halogens? It is because the wavelengths for halogens are all falling well below 200 nm. i.e in the low UV region. But the difficulty is that atmospheric oxygen will start absorbing below 200 nm. This will complicate the situation. Another problem is that because of the high ionization potentials of halogens they are less ionized when compared with metals.