Some places may be best kept quiet about, but the use of barrels is not a dark secret!
The reasoning for not broadcasting every single detail of every mine to the world is usually a combination of two reasons:
(i) the landowner may not permit access, hence publicity is likely to get the mine locked off; and/or
(ii) the mine may be damaged by repeated visits, artefacts robbed, graffiti drawn etc.
As a result, those mines that are particularly 'sensitive', either because of the landowner or because they're relatively untouched, tend to be those you don't hear as much about.
There may also be instances when somewhere is considered dangerous, so casual visitors are best kept out. There are examples all over the internet which prove that allowing badly prepared people into some mines is just asking for trouble.
One fatality and it may be that we all lose access to a lot of mines, so the risk has to be minimised.
It's nothing too exciting, rather that it's a mostly sensible policy.
To the newcomer, it may seem a bit odd, but generally people will be willing to show you the more commonly visited places and - ultimately - those that are a bit more out of the way.
Hello again darkness, my old friend...