The correct answer is option 2: The Swiss people continue to vote against joining the EU as they do not want to lose Switzerland`s sovereignty.

Option 1 states that few EU countries like France and Germany are opposed to Switzerland joining the Union. This implies that the opposition from these countries is the main reason why Switzerland has not been able to join the EU. However, this is not accurate. While it is true that there may be some opposition from certain EU countries, it is not the main reason for Switzerland`s inability to join the EU.

Option 3 suggests that the EU rejected Switzerland`s application because the Swiss Government did not agree to use the Euro as currency. However, this is not the case. The issue of currency is just one aspect of the negotiations, and it is not the sole reason for Switzerland`s non-membership.

Option 4 states that all the given options are correct. However, this is not true. Option 2, which highlights the Swiss people`s opposition to joining the EU, is the most significant reason behind Switzerland`s inability to join the EU.

In conclusion, the main reason why Switzerland has been unable to join the EU is that the Swiss people continue to vote against joining the EU to preserve Switzerland`s sovereignty.