Unfortunately, your(又はthe candidate's) reasons for applying for the post were weak and so on this occasion, your(又はhis/her)application was unsuccessful.
「志望理由」=reason for wanting to join the company/reasons for applying for the post
「志望理由が弱いため、不合格です」という文章はさまざまな英訳がありますが、少しソフトに行った方が良いと思います。
Unfortunately, your(又はthe candidate's) reasons for applying for the post were weak and so on this occasion, your(又はhis/her)application was unsuccessful.
※ candidate = 志望者, application = 申し込み, unsuccessful = 不合格、不成功
もう少しソフトに言いたいなら、「too weak」(=弱すぎる)の代わりに「unsuitable」(不適当、不適切,不適任)というもう少しあいまいな単語を使う事が出来ます。