Isn't there a lot of calls to violence on Twitter and Facebook? Why singling out Parler, they moderate calls to violence as best as they can too.
The difference is Facebook & Twitter take their content moderation (and it's overall impact on their business) very seriously.
From any angle you look at this, Parler failed at reasonably moderating their content. More so, at their own peril despite being warned by AWS about the violent content.
Parler got popular amongst Trump supporters and nothing draws negative press like Trump.
OTOH, their volumes did not justify the complex environment they allegedly had and they should be very well within the skill set of the very average devops engineer. No large scale deployment expertise required.