Benih gagal tumbuh bukan kerana baja, tetapi kerana saliran kurang baik.

because

Breakdown of Benih gagal tumbuh bukan kerana baja, tetapi kerana saliran kurang baik.

• tetapi: but
• bukan: not
• gagal: to fail

Questions & Answers about Benih gagal tumbuh bukan kerana baja, tetapi kerana saliran kurang baik.

Why is bukan used here instead of tidak?

In this sentence, bukan is used because the speaker is rejecting one reason and replacing it with another:

bukan kerana baja, tetapi kerana saliran kurang baik
= not because of fertilizer, but because the drainage is poor

A very common pattern in Malay is:

bukan ... tetapi ...
= not ..., but ...

Even though kerana is involved, this is still a contrastive structure, so bukan sounds natural.
If you used tidak here, it would sound unnatural in standard Malay.

How does bukan kerana ..., tetapi kerana ... work grammatically?

It is a fixed contrast pattern:

• bukan kerana X
• tetapi kerana Y

So the sentence is structured like this:

• Benih gagal tumbuh = the seeds failed to grow
• bukan kerana baja = not because of fertilizer
• tetapi kerana saliran kurang baik = but because the drainage was not good / was poor

This is directly parallel to English not because X, but because Y.

What is the function of gagal tumbuh? Why not just say tidak tumbuh?