Don't know the precise answer and a bit busy with other law things to look it up for you. However, I will point out a couple of basic Community law principles which are important as to the understanding of any potential reason why not.
1) It is possible for a State to derogate, avoid, the principle of the freedom to provide services, as well as the freedom to supply goods. They can do so under various provisions within the EC treaty which give a number of grounds including public policy etc. I suppose it could be the case that such restrictions have been justified on this basis before the ECJ.
2) However, I think a second possible explanation is more likely. Whilst there can not in principle be barriers, or measures with equivalent effect, to the freedom to provide services across borders, there can be restriction and regulation so long as it is proportionate. Such regulation may well be in the form of being a registered company within a Member State, having a registered office, having bank accounts etc. This would be particularly important in terms of sectors such as insurance where they could cut and run without paying up if they are not properly regulated.
If option 2 were to be the case it does not restrict the freedom to provide services within the common market per se, it merely regulates it. For instance, AXA (a French company?) has established itself in Ireland and is providing services. You are just unable to get a policy with them through their French office.
As I said, I don't know definitively, I am merely providing sound Community law principles on which it may be argued.