nihongo ga motto zyouzu ni naru tame ni, mainiti ganbarimasu.

Because 上手になる describes becoming good at something, not doing something directly to it.

In Japanese, the thing someone is good at is often marked with が:

• 日本語が上手です = be good at Japanese
• 日本語がもっと上手になる = become better at Japanese

By contrast, を is used with verbs that directly act on something:

• 日本語を勉強します = study Japanese
• 日本語を話します = speak Japanese

So in this sentence, 日本語 is the thing the speaker wants to become better at, which is why が is natural.

もっと means more or even more.

Here it modifies 上手に/上手になる, so the idea is:

• 上手になる = become good
• もっと上手になる = become better / become more skilled

It comes before the word it modifies, which is normal in Japanese.

上手 is a na-adjective.

When a na-adjective is used before なる to mean become ..., it usually takes に:

• 静かになる = become quiet
• 元気になる = become well / energetic
• 上手になる = become skilled / get good

So 上手になる means to become good at something.

In this sentence, 日本語がもっと上手になる means to become better at Japanese.

なる means to become.

So:

• 上手になる = become good
• もっと上手になる = become better

It is in the plain, non-past form because it is part of the pattern ～ために. It does not mean the becoming is happening right now; here it expresses the goal or purpose.

This is very common Japanese grammar.

In a sentence with a main clause and a subordinate clause, the subordinate clause is often kept in the plain form, even when the whole sentence is polite.

So:

• 日本語がもっと上手になるために = in order to become better at Japanese
• 毎日頑張ります = I will do my best every day

Only the main sentence ending, 頑張ります, shows politeness here.

This is normal and natural Japanese.

Verb dictionary form + ために means in order to ... or for the purpose of ....

So:

• 日本語がもっと上手になるために = in order to become better at Japanese
• 毎日頑張ります = I will work hard every day

Together, the first part gives the purpose, and the second part gives the action taken for that purpose.

This is a very common structure in Japanese.

They are doing different jobs.

1. 上手に

   • Here に turns 上手 into the form used with なる
   • 上手になる = become good

2. ために

   • Here に is part of the grammar pattern ～ために
   • なるために = in order to become

So even though both are に, they are not the same grammatical use.

Many time expressions in Japanese do not need a particle.

So 毎日頑張ります is perfectly natural and means:

• every day, I will do my best

This is similar to other common time words:

• 今日勉強します = I will study today
• 毎朝走ります = I run every morning

Some time words do take に, but 毎日 usually does not.

頑張ります is the polite form of 頑張る.

It often means:

• do my best
• try hard
• work hard
• keep at it

In this sentence, it does not necessarily mean physical effort. It can mean the speaker will keep studying, practicing, and putting in effort every day.

So 毎日頑張ります is something like:

• I’ll work hard every day
• I’ll do my best every day

Yes, it is omitted, and that is very normal in Japanese.

A full version could be:

• 私は日本語がもっと上手になるために、毎日頑張ります。

But if the speaker is obvious from context, Japanese usually leaves 私は out.

This makes the sentence sound more natural in many situations.

The comma is optional and mainly helps readability.

Since 日本語がもっと上手になるために is a longer purpose phrase, the comma gives a small pause before the main statement:

• 毎日頑張ります

You could also write the sentence without the comma, and it would still be correct:

• 日本語がもっと上手になるために毎日頑張ります。

Both are natural.