It's done to avoid a potential conflict of interest.
Your own solicitor is contractually obliged to act on your behalf, and to have your interests at heart, in much the same way that a solicitor representing a client in a criminal case does. (i.e. the solicitor in a criminal case is obliged to take his/her clients' word for things and to put those representations before a court, even if he/she doesn't actually believe a word of what the client says. Similarly, your solicitor is obliged to take your word when you explain the source of your finance, even if he/she actually thinks it's possible that you might be laundering drugs money).
So an independent solicitor has to be called in to, in effect, act on behalf of the Crown, in checking that everything really does comply with the requirements of the law.