Why did democrats filibuster the Grassley-Cruz legislation that would have stopped the church shooter?

https://www.westernjournal.com/cruz-dems-filibustered-legislation-landed-shooter-jail-instead-murdering-churchgoers/
Update: https://www.c-span.org/video/?312207-1/republican-senators-gun-legislation
Update 2: http://www.breitbart.com/video/2017/11/06/cruz-democrats-filibustered-legislation-that-would-have-resulted-in-texas-church-shooter-being-in-prison/
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