Code that would show the countries with a greater GDP than any country

I am new to SQL and was trying to solve a question on SQLzoo

Select the code that would show the countries with a greater GDP than any country in Africa (some countries may have NULL gdp values).

My answer to the question was

    SELECT name FROM bbc
    WHERE gdp > ALL (SELECT gdp 
                     FROM bbc 
                     WHERE region = 'Africa' 
                     AND gdp<>NULL)

But the correct answer on the site is

SELECT name FROM bbc
 WHERE gdp > (SELECT MAX(gdp) 
              FROM bbc 
              WHERE region = 'Africa')

I am not getting why the answer selected by me is wrong

Quiz Question No 5

Answer

The problem with the > all (...) solution selected by you is the fact that some of the gdp values is null.

When you compare a non-null value with a null, then the result is null, unless you use a null-safe operator, such as is null.