I believe that has been happening - Japan, the USA, and various other countries have made it clear that trade deals with the UK would be forthcoming and welcome... but the trouble is that our ability to make any new trade deals depends on the EU-UK Withdrawal Agreement, as well as the Future Relationship (i.e. the future trade deal with the EU, that also cannot be negotiated prior to Brexit Day), that at this point in time will pretty much prevent the UK from making comprehensive new trade deals with the rest of the world. The EU have gone a step further and demanded that, irrespective of what the UK-EU's future relationship might turn out to be, that the UK will commit to a 'level playing field' and not be allowed to strike trade deals with third countries that give the UK a competitive advantage over the EU i.e. basically the same constraint that existing EU member states must abide by.