Because この is already a determiner that directly modifies a noun, like this in English.
So:
You do not say:
The particle の is used in other kinds of noun modification, but not after この / その / あの / どの.
は marks the topic of the sentence.
So in:
この植木鉢は大きい。
the topic is この植木鉢 = this flowerpot.
A natural way to understand the structure is:
This is why は is often translated as is in simple sentences, but it does not literally mean is. Its real job is to mark what the sentence is about.
Japanese sentences often place the main predicate at the end.
In this sentence, 大きい is the predicate, so it comes last:
So the basic Japanese pattern here is:
topic + は + predicate
That is different from English, where big usually comes after is:
There is no separate word for is in this sentence because 大きい is an い-adjective, and い-adjectives can act as predicates by themselves.
So:
This is very different from English. In English, you need is:
In Japanese, with an い-adjective, you do not need だ or です for the basic plain form:
A polite version is:
It is an い-adjective.
Japanese い-adjectives are adjectives, but they behave in some ways like verbs because they can end a sentence directly.
Examples:
So grammatically, it is still an adjective, but it can serve as the sentence predicate without needing a separate is.
Yes, you could say:
この植木鉢が大きい。
But the nuance changes.
In a simple neutral statement, は is very natural.
Using が is possible, but it usually feels more specific or contrastive depending on the situation.
Japanese normally does not use spaces between words.
So the sentence is normally written as:
この植木鉢は大きい。
Spaces are often added in textbooks or beginner materials to make the structure easier to see.
That means:
A common polite version is:
この植木鉢は大きいです。
This adds です after the い-adjective.
So:
Both mean the same thing, but です makes it more polite.