About "had"

[嘟嘟嘟嘟]

a) They did not encourage the development of tourist-related activities until recently.
b) They had not encouraged the development of tourist-related activities until recently.

I think sentence a is right. But why can't we use the past perfect tense like sentence b since "recently" refers to a point in the past?

[Rover_KE]

Where did you encounter those sentences, 嘟嘟嘟嘟?

(How do you pronounce that?)

[tedmc]

The past perfect tense is used if we need to distinguish an event in relation to another event or point in time in the past, which the past tense cannot. In this case, the sentence is clear using the simple past tense, so there is no need to use the past perfect tense.

[嘟嘟嘟嘟]

Where did you encounter those sentences, 嘟嘟嘟嘟?

(How do you pronounce that?)

They are just from an English test paper, asking students to make a choice about the tense (did or had) in the sentence.
If "recently" is a point in the past, "they did not encourage..."happened before "recently", then why can't I say "they had not encouraged...'?
嘟嘟嘟嘟 is pronounced similar to /du: du: du: du:/

[Piscean]

why can't I say "they had not encouraged...'?

You can.

[Tdol]

We often don't bother to use the past perfect, but that doesn't mean it is wrong to use it. It's fine here.