Bullpups Are Futuristic

A "bullpup" rifle is one where the action is behind the trigger. This allows for a longer barrel (which means more accuracy) with shorter overall length and weight, but increases the complexity adding to manufacturing time/cost as well as increasing the chance of malfunction and making maintenance more difficult. Further, in most cases this means the rifle can't be ambidextrous without making it eject spent brass downward (adding further complexity). As the British, French and Australian armies have discovered this is also an issue for right handed users as it prohibits shooting left of cover which is a major disadvantage in today's close range, urban combat.

Why bullpups are seen as futuristic is unclear. Perhaps it is that they are often made with sleekly angled (so that it will not snag on a soldier's gear) plastic or that many of them have integral optics and less prominent iron sights (a tie in with Sighted Guns Are Low Tech). The prominent ventilation holes, necessary for the barrel to cool effectively, might also play a role. The Mini-14 in aftermarket bullpup kits is an exceptionally prominent example in live-action works due to it keeping the Mini-14's high tolerance for blanks yet not maintaining the Mini-14's exceptionally plain or ugly (depending on configuration) appearance.

For whatever reason, it has been decided that bullpups look futuristic. Off the shelf bullpup conversion kits for most popular rifles (The AR15 is one of the few exceptions as its buffer tube renders it incompatible with such conversions) are often so commonly used as futuristic weapons in movies and film they have their own sections on the Internet Movie Firearms Database under their parent firearm.

Anime and Manga

 * The fictional Colt M72A1 bullpup is the Earth Federation's primary small arm in Gundam's Universal Century timeline. While it's never named outside of production or supplemental materials (due to the franchise's focus on Mecha and not small arms), the design has been consistent since at least Mobile Suit Gundam 0080: War in the Pocket where the production art merely refers to it as the "Earth Federation Forces Official Assault Rifle".

Film

 * Soldier
 * Starship Troopers has the fictional (but with several of the props actually functional modified Mini-14s) Morita rifle series. Like most of the props from the movie they have been reused in countless future productions.
 * Mini-14s in after-market bullpup stocks are further modified to be the primary weapon in Total Recall

Live-Action Television

 * Bullpups of all kinds appear in Firefly with the intent of appearing futuristic.
 * The Star Trek: Voyager episode Nemesis features the heavily make-uped yet human bodied Kradin aliens using AK derivatives in aftermarket bullpup kits.
 * Stargate SG-1 made extensive use of the FN P90.

Tabletop Games

 * The Elysian Drop Troops (space paratroopers) of Warhammer 40,000 use a unique pattern of bullpup-configured lasguns, so that their weapons are shorter, lighter, and more maneuverable. It makes them look more advanced and modern, in comparison to the very archaic and Used Future look of typical regiments.  There's no engineering reason that all lasguns shouldn't be this way, since they don't shoot physical bullets and so there's no additional complexity to the design -- it's just a matter of routing the power to the middle of the weapon versus the back -- but maybe the Guard sticks with the "typical" pattern to maintain familiarity between regiments and worlds that may use bullets as well as lasers.