Part of the reason is that the channels are created with agreements with a number of other providers in HD formatted material.
Each channel doesn't have VOOM produced content, just content VOOM negotiated for to be placed on their channels. When VOOM sells their format to another company, the agreements do not necessarily "transfer" and E* would have to "renegotiate" with all of those different providers to recreate their own Rave HD and Monster HD channels.
Way to much work and not necessarily "cheap". Voom's pricing was per customer and with 26,000 subscribers wasn't very expensive. Up scale that and involve negotiations into the mix, probably with some providers E* played hardball with before, and you get the picture.
Logistics nightmare.