As @vicky_molokh wrote, Infinite Worlds includes power struggles between world-hopping organizations, and so conspicuously using high technology may likely tend to attract their attention, so using a gun, tank or laser can tend to backfire by attracting multiple very aggressive types who also have such technology and you will have given away that you're such a person, whereas they may be carefully maintaining their local disguises until they strike.
But I'm writing to add that also, players and speculators often greatly overestimate the dominance they could get by bringing a few higher tech weapons into a lower tech setting. Largely because they tend to greatly underestimate the intelligence and cunning of the people in the lower tech setting, and because they don't really think through the full situation.
For example, a machinegun will certainly let you kill some people and have a great effect... until you run out of ammunition, or it jams. And an SMG might not even be that great against people with armor and shields, also because they're not very accurate.
But far worse than all of that is that the people you hope to dominate will tend to include very smart and crafty people, and using a super weapon will attract many such people's attention. They will want your super weapon, and they will use their intelligence to get it from you, one way or another.
In gaming such situations, I've seen them end quite quickly in various ways, including stealth, night attacks, backstabbing, poison, surprise attacks, superior numbers, magic (Reverse Missiles trumps high-tech ranged weapons), and various deceptions.