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.