I imagine the basis for this is that, by the time your IVA has been accepted, your IP has put in a lot of work and is owed the fee for this with the expectation of an ongoing income. If you ditched that IP you would need to make up the loss first. Then there would be further costs to both the incoming and the outgoing IP's in transferring the accounts, re-appraising creditors (who, as party to the agreement, would also have to give their consents). Naturally the incoming IP would have expense that would have to be met in setting up new accounts. And this is all assuming that their working practices are compatible.
It is, by no means, a simple business, but, en bloc, when a whole book of cases is transferred the costs are mitigated by economies of scale. Plus, of course, a transfer brought about by the IP is, in certain circumstances, catered for in the legislation.
All that said, I believe that there should be some mechanism whereby, in certain circumstances, a transfer should be possible when the relationship between client and IP has broken down. This would make certain firms more client friendly.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014