YOU SAID:

Cupcakes are delicious, and this is why.

INTO JAPANESE

カップケーキがおいしい、これが理由。

BACK INTO ENGLISH

Cupcakes are tasty, this is the reason.

INTO JAPANESE

カップケーキがおいしい、これが理由です。

BACK INTO ENGLISH

So, hence this.

INTO JAPANESE

これが理由です。

BACK INTO ENGLISH

So, hence this.