I blame the ACLU.

[Soul Crusher]

http://frontpagemag.com/2012/dgreenfield/aclu-killed-connecticut-forcible-institutionalization-law-that-might-have-prevented-killings

[The True Adonis]

Lets not.  The ACLU did a good thing here as this would have been another law we do not need.  Furthermore, the ACLU has done a lot to protect Gun Rights, arguably moreso than the NRA.

Check this out.
http://www.aclu.org/racial-justice_prisoners-rights_drug-law-reform_immigrants-rights/second-amendment
We do not, however, take a position on gun control itself. In our view, neither the possession of guns nor the regulation of guns raises a civil liberties issue.

[Al Doggity]

Not to mention, there's no evidence anyone, at any point, tried to have Lanza committed. Whether he even had clinically diagnosed mental problems is still a matter of debate, as of right now. He lived with his mom. The guns belonged to her and some reports have said she  trained him in gun usage.