Why IPS officers not interested in DIG postings ?

In the Central Armed Police Organisation, many DIG posts for IPS officers are vacant. In CRPF out of 19, only one IPS officer is posted as DIG. Remaining 18 posts of DIG for IPS quota has temporarily diverted to CRPF cadre officers. CRPF has 37 DIGs posts. In BSF also out of 26 DIG posts 15 IPS quota posts has temporarily been diverted to BSF cadre officers. In SSB also 7 IPS quota DIG posts are diverted to SSB cadre officers.