There is a variety of reasons why conversion alone may not be sufficient in particular cases. There will be gaps where some areas of converted law will be entirely unable to operate because we are no longer a member of the EU. There will also be cases where EU law will cease to operate as intended or will be redundant once we leave. In some cases EU law is based on reciprocal arrangements, with all member states treating certain situations in the same way. If such reciprocal arrangements are not secured as a part of our new relationship with the EU, it may not be in the national interest, or workable, to continue to operate those arrangements alone.

High

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/604516/Great_repeal_bill_white_paper_accessible.pdf

Chapter 3, Paragraph 3.3