They probably took it off air for the same commercial reason that many programmes disappear from TV channels.
MTV (who own Viva) have to pay to buy the broadcasting rights. Independent auditors monitor the audience figures for the programme. MTV try to sell enough advertising to cover their costs (and to make a profit) but, if the audience figures aren't high enough, advertisers won't pay the rates required to meet the cost of buying the programme. So (since they can't afford to make a loss) MTV decide not to buy further series of the programme.
For a definitive answer, email MTV:
http://www.mtv.co.uk/contact
Chris