The universe in reality isn't neatly sliced into increments of ten, why should jump travel be neatly sliced into similar nice, round numbers? Earth's orbit is not neatly 365 24 hour days, hell, our rotation isn't even a full 24 hours.
It's all got variables and variations. 30LY is an average because existence doesn't (and shouldn't) come in neatly precalculated packages.
It's not Exactly 30 LY, because the universe doesn't come in exact scaling measurements. It's all built to tolerances, with variation and variables. your car doesn't get Exactly the mileage (Kilometerage?) that it says on the sales brochure, that's an average worked out in testing by taking a lot of samples, and dividing the sum.
so too, I would suggest, for Jumpship travel. Some Merchants can stretch further, call 'em "Wednesday Cores", while others fall short (Friday and Monday cores)-but they all can just about reach 30 LY, more or less.
It's the floor, below which, the product is too defective to sell.
But they're not going to promise you 31, 35, or 40 LY at the dealership, because not all of 'em can do that...but at the shipyard dealer, they won't sell you a core that can't make 30LY out of the box once that became the standardized expectation-they'll grind it up and reprocess it instead until they can sell you a core that will MAKE 30 LY...even if you're only going to ever make a 5LY hop between two close-knit stars.
Because they can sell it to at a given price point (well, sell it to the bank who'll repossess it from you if you can't make your payments.)
It's like when GM would built a model of car, and some of the cars off the line can pull 47 miles/gallon out of a downdraft carb, but the average is going to be 15-that's what they put on the sticker, and any of that model that can't make 15 gets reworked, because if it gets out that they're selling bum vehicles, banks won't do the financing.
Basically, for a Standard Jumpship then, 30 LY is the average range that the shipping industry settled on for "we can finance this at a good rate". Further WOULD be better, but it's not reliable enough to keep the yard working, so they don't claim to tolerance or manufacture past 30 LY. (after which, you start getting weird accidents, see for example, SLS manassas).
So why 30LY? it's a decent chance in a unvierse like Battletech, that there's going to be habitable something within 30LY of a given star, and enough of them WERE habitable that this became a baseline expectation for "We're probably going to go this far in a single jump at least once, usually more than once".
which lets the manufacturer have tolerances that are reasonably repeatable with the core-casting or core-forging gear they have.
and it covers most potential clients' needs.
This, in turn, would influence MILITARY suppliers (compact cores, subcompacts) because if your fleet's movements aren't predictable, (aka you don't know how far any given ship of a given class can actually go reliably) then your navy's not going to be much good, and being able to jump as far as Bill McHukster in his Merchant makes your space soldiers a laughingstock when they come to enforce the rules.
and nobody wants that.
can you find a ship that can go further on a jump? with enough money, but you're not going to find two that can do it with any reliability minus being the conglomerate that owns all the merchant banking and the lion's share of communications...and that conglomerate or goofy cult? has a vested interest in maintaining the appearance of 30LY being an absolute, as opposed to average, limit.
after all, they own parts of everyone else's yards too, and there's a decided financial and strategic motive to keep things...stable...ish.
