First of all, congratulations on getting a Traveller game up and running with some new players!
Here's my opinion on this issue. Of course it's based on my experience with D&D players I've interacted with, and it's not meant to state that everyone is like this:
Part 1 - The Problem
D&D teaches a lot of bad habits, and it's difficult for people accustomed to them to overcome them, especially if they don't see anything wrong with them.
These habits include:
* Addiction to the drip-drip-drip of incremental level rewards and the nonsensical powerups they bring.
* D&D's game mechanics make the fastest way to get those incremental rewards is to kill as many people and animals as possible. It encourages players to treat the game setting and everyone in it as objects for them to destroy to get their next experience point fix and loot drop.
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In Traveller, this way of thinking creates big problems when you have NPCs respond reasonably to trouble the player characters are causing. The players beat down a bar full of patrons, then shoot the local cops who respond to the disturbance, then shoot the local tactical team (if there is one), and then they hop back across the extrality line chortling as they go. When starport security attempts to arrest them in response to a local extradition request, the PCs try to blast their way out of Mos Eisley. If they die, they get upset. If they get away, they're outlaws who have to stay one jump ahead of the authorities, but that doesn't last long. Then they die, and get upset. When you ask them what they expected to happen when they acted the way they did, it comes back to the string of easy consequence-free victories they're used to from playing D&D. And the cruelest cut of all is that they didn't even get any experience points.
* The game mechanics encourage the players to treat the game setting as disposable, since it's only there for them to have a plausible narrative and some nice scenery while they get on with the serious business of killing people and leveling up. This means they're accustomed to treating your worlds, plots, and NPCs are disposable stage props that only exist to gratify them. Often they have trouble taking anything seriously, and it's all a big joke to them. Example: I convinced some people to play a Traveller session and the planet the players were on had an unbreathable atmosphere and daytime was extremely hot. The population lived in massive ziggurat arcologies. Now, all the players had to do was figure out how to get hardened vacc suits (like rent them) and watch the time when they went outside to deal with an organized crime faction. Simplicity at its best, right? Wrong. They got upset, and said dealing with the environment was too much of a hassle when they just wanted to go outside in shorts and t-shirts, kill everybody, and then leave.
* D&D's game mechanics discourage logically thinking things through. Players expect things to work the same way in other games and don't want to make the effort to think through situations. It seems like extra work for no reason to a lot of them. Examples: A fireball explodes in a 4'x4' prison cell. A character makes his saving throw and mysteriously, inexplicably "takes half damage", no logic allowed. A character gets bitten by a rattlesnake. He makes his saving throw, so he's fine. No pain, no being bedridden screaming in agony. No logic there either. When players bring this attitude to Traveller, there's a big mismatch between their expectations and how Traveller works. It doesn't matter what big tough guys their characters are, a landmine will still rip their legs off, and then they get upset because in D&D they could take crossbow bolts to the naked eye and just shrug it off.
* D&D's game mechanics encourage rules-lawyering to get the best min/max build in order to kill as efficiently as possible. Stacking abilities, feats, items, etc. to make the biggest Monty Haul murdertwink imaginable is almost a game in itself that a lot of D&D players really enjoy. One player brought all his books in on a handcart so he could prove that every one of his feats and items was legit. This is completely untranslatable to Traveller, but the impulse manifests as players trying to get the deadliest weapons, ships and gear they can without even a threadbare reason why. When you say no, they get upset.