Pretty much a choice. It comes down to two things: the wax they use, and the fragrance 'load.'
Soy is typically best and provides the best 'scent-throw' (i.e. how far in the house it will spread). But lots of big sellers don't like to use soy as it's tricky to work with and quite expensive. It also really needs to be hand-poured and have the temperature carefully maintained etc, otherwise the risk of 'frosting' is higher. And whilst frosting is normal, it's not pretty to look at. Therefore, lots of big companies (i.e. that don't hand-pour their product) will use parrafin (Jo Malone and Diptyque do) which can give a slightly clinical smell underneath the fragrance - that's probably what you've noticed. Parrafin wax is also incredibly bad for your health. Soy wax is not.
So that's the first corner big companies cut that makes the product worse.
And then the fragrance load is the trickiest bit. Under legislation you can have a maximum of 10% of the product being 'fragrance oil.' So if you have a 100g candle, 10g of that is fragrance. The more fragrance that is used, the stronger the scent. Lots of places (Jo Malone, Dyptique) etc, use about 8%. That's because the more fragrance, the higher the cost. Fragrance oil is more expensive than wax. Some can cost £70 for 1kg. But, if you use too much fragrance oil, it can become overpowering and you also lose part of the scent.
A good example of the above is Jo Malone's grapefruit candle gets lots of negative reviews because not only is it parrafin wax (which gives a horrible after-smell) they also mucked up the fragrance load and poured their latest batch too strong. So they've got a tonne of reviews complaining about the shitty smell (the parrafin wax) and overpowering grapefruit - which has happened because they poured too much fragrance oil and therefore the other scents within the scent were lost.
Candle making is really difficult (which is why even though I know a tonne about it, my business decided to stick to soaps) when done properly. It's why big companies aren't necessarily that reliable. Ultimately, hand-pouring with soy wax will always yield the best results that mass produced parrafin blends.
@AndeanFlamingo - the above might be helpful for you as well.
Parrafin Wax