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, range and bullet velocity) 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 and firearms, and often lasers, in Sci Fi will be ones with bullpup designs. Off the shelf bullpup conversion kits for most popular rifles 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 starting in season 4 to the point it become iconic of the series. Interestingly the Stargate Program is the niche the P90 could never find in the real world: Its small size and light weight makes it suited for users that aren't primary combatants (which is the case for most SG teams) yet has excellent armor piercing ability (noted by O'Neil when the P90 first appears) needed against the often heavily armored Jaffa.
 * From a production point of view its downwards ejection greatly simplifies actor placement (no need to worry about ejecting brass onto someone), its ambidextrous controls make it usable by any actor and the light weight is also appreciated by actors who have to hold the thing.

Tabletop Games

 * Warhammer 40,000, among the many variations from vintage wooden rifles to blocky things with folding stock, has bullpup-configured lasguns. The Elysian Drop Troops (space paratroopers) are equipped with these (Accatran pattern), 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. Possibly because the Guard often uses bayonets, and that's where total length is a good thing.
 * Warhammer 40,000 Roleplay has a good assortment. So there are generic Bullpup Lasgun (stats between a common lasgun and lascarbine without advantages of either), Accatran MkIV lasgun used by the Elysians (like a common lasgun, but with bonus on "who's first" opposed tests and range reduced almost to carbine's) and Roth “Lightning” lasgun (long-barreled version with longer range than common, but the same weight).