Because they are not impartial. Without details it is hard to be specific, but here are some examples:
Say for instance, you had a large amount of debt, but a great disposable income, they could suggest a Debt Management Plan. You could pay £120 per month towards your debts, but if they are taking the first £40, only £80 is going to your creditors, meaning that it takes longer to get paid off.
With regards to IVAs, they make an offer to creditors based on your available payments. However, this is reduced by their fee. Usually £5000-£8000. The offer is based on a pence in the pound basis. So if debt is £1000, and they offer 40p in the pound, the creditor gets £400, and writes off the remaining £600 as bad debt. However, in order for an IVA to succeed, it has to be worth the creditors' while.
E.g.
Debts of £30000, available payment £250 per month. Usual length of IVA is 5 years. If that all went to creditors, they would eventually get £15000 (£250 X 12 X 5). This means that they get a payment of 50p in the pound, and write of £15000. However, DFD need their £5000, so they take that off, and the creditor is instead offered 33p in the pound (£10000 overall). Therefore, the creditor may vote against an IVA, because it is below their acceptable threshold.