Simple explanation. Freeview, Freesat, Sky & Virgin are all different commercial "platforms". The channels owners pay the platform provider to join that platform, so each platform joined is an additional expense. Therefore channels will appear on some platforms, but not others. Also, with "pay" platforms like Sky & Virgin, channel owners can opt to take a cut of subscription income from the channel pack they join. This is why Horror channel appears on Freeview, but is not on TV M (Virgin's free equivalent package).
Only the basic PSB (Public Service Broadcast) channels (BBC, ITV1, CH4 & CH5) must be provided free on all platforms. All the others are subject to commercial agreements with individual platforms..
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV+), Talk Unlim Telco, VIVID 100, Virgin PAYG Mobile