The problem, as both Motherboard and MIT Technology Review have recently pointed out, is based around how Android works. Rather than being a product that's worked on in a lab at Google and then pushed out to all users, the operating system is forked, tweaked and altered by every company, and carrier, that uses it. That means that, instead of one version of the software, there are hundreds of minor variations, which makes universal security patches all that more difficult to distribute. That's why the Stagefright bug is so terrifying, since it threatens anything up to 950 million devices with little chance of a one-size-fits-all solution being created. We've reached out to LG so that we can confirm that it will commit to monthly security updates, but have not had a reply by the time of publication.
Update: LG has reached out with the following statement:
"LG is committed to bringing its customers the utmost in device security. Toward that goal, we are now starting to roll out updates for in-market LG devices potentially vulnerable to Stagefright. As an additional step, LG will be providing security updates on a monthly basis which carriers will then be able to make available to customers immediately. We believe these important steps will demonstrate to LG customers that security is our highest priority."