The reason is because you have not done well in it recently and you have lost interest.

Question

这是个改错题，为什么要把because改成that，可不可以直接去掉because？

Answer 1

reason与because重复，故需要改掉。
is后面接名词或宾语从句，that为宾语从句的代词

Answer 2

不用改吧，that特指that从句 可有可无

Answer 3

because与reason不能同用，句式：the reason is that...