The “there be” existential construction vs. the passive reflexive construction

The “there be” existential construction vs. the passive reflexive construction
 

¡Hola!


I have a sentence in English to be translated into Spanish:
There were a lot of amusement parks built in southern California in the twentieth century.

I translated it as:
Había muchos parques de atracciones construidos en el sur de California / la California meridional en el siglo XX.

The key has:
Se construyeron muchos parques de diversiones en el sur de California en el siglo veinte.

I would translate the key sentence as
They built a lot of amusement parks in southern California in the twentieth century.
A lot of amusement parks were built in southern California in the twentieth century.

My thoughts regarding my rendition of the original sentence:

1. the imp. Vs. the pret.
The imperfect invites elaboration/amplification.
The preterite closes the subject / topic complete and done with.

As there is nothing to expand on the above sentence, I should have put Hubo muchos parques…

2. The “there be” existential construction vs. the passive reflexive construction
I believe that both constructions introduce new information (parks).

Is there a reason that the passive reflexive be preferred over the existential sentence?

Thank you.

There were .... built

implies both existence and the act of them being built. There is no suggestion the parks built themselves nor information about who did build them.

An amusement park needed to be built in order to exist is not something to be informed, as amusement parks aren't natural, so

había muchos ... construidos

means había muchos plus stating the obvious

Se construyeron

is the natural translation when you wish to highlight their existence and not who built them.

A different thing is when you try to say

Nadie sabe cuándo fueron construidos ni quién los hizo

In this case "se construyeron" would sound awful because it's sort of agentless, so to speak

Thank you, aleCcowaN. I very much appreciate your explanation.