First, lets look at the Longshot's history. We travel back in time to 2007 when the Longshot made it's shelf debut in it's dark blue format. It would be the last N-Strike blaster with the Blue/Yellow color scheme, as the Recon would change things over to Yellow (mostly). It was a large blaster, the longest N-Strike blaster at that time and sold for around $35. It was the first N-Strike blaster to introduce the removable barrel system and the Clip System, both of which have become defining traits of the N-Strike lineup. While the Longshot was a trendsetter, it didn't catch on very well at first. Hasbro failed to produce Streamline dart refill packs when it first came out, making spare ammo for it scarce. Thankfully, the Longshot initially included 2 clips and 12 darts, but it was still unfortunate to be limited to that. Once the Recon came out, Streamlines began to be sold in refill packs (though they sold out in many stores initially before they could be restocked). So in general, the Longshot kind of had a bumpy initial release that was overshadowed by the Nerf Recon.
Folks often point to the Longshot's potential as a powerful direct-plunger blaster as it's reason for being discontinued. While Nerf has not released a direct-plunger system that has been as large as the Longshot's since then, they still have produced Direct-plunger blasters since the Longshot's discontinuation. The Stampede, Big Bad Bow, Swarmfire, Jolt, and even the older NiteFinder are still on shelves with their direct-plunger systems, so the thought that the Longshot was discontinued for this reason doesn't have much weight to it.
Many blasters that are readily available on US shelves are not sold internationally. The Longshot was one of those US-only blasters until 2010, so it hasn't been discontinued. It is still sold in Australia, Singapore, and England and has been popular there. Though every blaster's discontinuation is inevitable, the Longshot at least has more production to go through overseas. International fans of Nerf finally get to enjoy what was readily available in the US for 3 years with 3 different color variations (Blue, Yellow, and Red Strike).
So, in a nutshell, the Longshot is gone because there's dozens of other Nerf blasters to choose from. With a flooded market, there's just no room for the beast that we know and love. So save your e-mails and Facebook messages to Nerf about bringing the Longshot back and enjoy the blasters that we have now that other fans across the sea cannot get.