Questions & Answers about zhè shì shēnsè yīfu.
Because 这 is acting as a pronoun, meaning this.
The structure is:
- 这 = this
- 是 = is
- 深色衣服 = dark-colored clothing / dark-colored clothes
So the sentence is literally This is dark-colored clothing.
You use 这个 when 这 directly modifies a noun and needs a measure word:
- 这个人 = this person
- 这件衣服 = this piece of clothing
But in 这是深色衣服, 这 stands alone, so no measure word is needed.
For the same reason: 这 is not directly attached to a noun here.
Compare these two patterns:
- 这是衣服。 = This is clothing.
- 这件衣服。 = this item of clothing
In the second pattern, 这 modifies 衣服, so you need a measure word such as 件. In the sentence you gave, 这 is the whole subject by itself, so no measure word appears.