The ITV companies HATE BskyB and do nothing to support the platform or the subscribers(Understandable). It took them three years just to put ITV1 onto SKY D and then only after pressure from the advertisers.
ITV2 was withheld from SKY because at the time OnDigital/ITVDigital was a going concern and they needed every edge they could get over SKY.
ITV2 only came to SKY because of the deal that brought ITV1 to SKY D, SKY agreed to pay £3 million (I think) to ITV for ITV2 and agree that it would be in a subscription package and ITV agreed to pay the price SKY was asking for EPG managment (postcodes etc).
With ITV2 being a subscription channel on SKY D it means they get some income from subscribers and it still gives DTT an advantage for potential consumers who are undecided if PAY TV is the way to go.