Point blank range and agility
"Consider which is faster, the Queen Mary, or a rowboat?"
Worst. Explanation. Ever.
I think - I hope - we can all agree on that. The Queen Mary is stuck in an ocean, the water resisting the ship's effort to bring her powerful engines to bear in a new direction. Get her out of the water into space, give her inertial compensators and decent attitude jets, and Her Majesty can spin like a rowboat. As for that rowboat: how quickly it can change its heading becomes irrelevant unless it can also apply enough acceleration to, in essence, get out from under the incoming hit by making its precise location fuzzy to a distant attacker. Doesn't matter how fast you spin if you only end up taking the hit on the aft deck instead of amidships. It's thrust that makes your precise location uncertain.
(How your precise location can be uncertain at ranges of under a light-second, when you glow like an infrared firefly against the backdrop of space, is a separate issue. We rather need it since otherwise the only thing that matters is how good we are at pointing the laser, which eliminates agility as a factor at ranges under about a half light-second. Traveller went with the sci-fi hard-to-hit-ships trope because it's quite a bit more entertaining than see-him-kill-him, so we'll stick with that.)
So, what might be a better explanation? And, while we're at it, what about another seemingly unrelated problem?
From time to time, one looks at the personal combat rules and thinks, "Wow, if you could get a fighter to these ranges, you could really mess up a dreadnought." Lasers and energy weapons have double the penetration at ranges under 5 km (though atmosphere plays with that a bit; I may come back to that). That alone wreaks havoc, because even a maxed out armored dreadnought finds itself losing maneuver and weapons, and any lesser creature sees interior explosions and criticals become a possibility. But, there's more: time scale is 1/200th the space combat time, and lasers and energy weapons can potentially fire 30 times in a minute. If ships close to personal combat ranges, the rest of the battle stops while the two wrestlers swiss-cheese each other with their lasers.
Why doesn't that happen?
(Rationalization time!)
Perhaps because that miracle of future technology, the maneuver drive, behaves like a spinning top as your speed increases: the faster you go, the more effort it takes to change the orientation of the drive (and, by extension, the ship it's mounted to). Whatever magic it uses to interact with space to provide thrust has that one weakness: vector inertia. At speeds of a couple thousand kilometers per hour, the effect is insignificant, but at space battle speeds - a minimum of 75,000 klicks per hour - it can take a minute or two to manage a 180 degree turn, and the faster you go, the harder it gets.
This turns combat for ships at under 12,000 Km range into something that looks very much like the old Star Fleet Battles game: slower ships make little movement but turn quickly, inside the radius of their faster opponents. With constraints on the ability to turn based on your velocity, getting and staying close to someone pretty much requires their cooperation, so ships have no particular problem staying out of the 5 km kill zone, and what they do when they're both in the same hex becomes no different from what they'd be doing if they were in neighboring hexes.
How does that play into agility? Well, we need to be careful here. If we say added agility allows you to turn faster, then we're back to the more agile ship using that agility to turn inside the less agile one despite their higher speed, and we're in the swiss cheese factory again. No, in this case we take a page from the Starship Operator's Manual: for very brief periods, additional energy can be fed into the drives to increase the drive's acceleration by up to 400%. Since your drive can offer a bit of thrust in any direction (25% lateral, 10% reverse), you become a hummingbird, your vector gaining a quick burst left, right, forward, or back. Your overall vector remains predictable, but your precise location along that line becomes more difficult to predict. Unlike the normal operation of the maneuver drive, this function varies with the mass of your ship: heavier ships require more energy to alter their base vector beyond the normal rating of the drive. You are also still constrained by the limits of your inertial compensators, so the effect has an upper ceiling, but you become (within the Traveller universe convention of hard-to-hit ships) a harder target.
Oh, and you don't get to 5 km range unless the other guy cooperates or is dead in space.
