In what way do you think is it far more difficult for customers to get to?
Because for most people it will involve taking a minimum of two modes of transport whereas they can take one at present, and if they drive it will be difficult to find parking.
At the moment, all trains, trams and most bus services take you to within a couple of minutes' walk of General Register House, and there is parking close by in the St James' Centre.
Yes, there is Edinburgh Park Station, but (stating the obvious) the only trains that stop there are the ones that happen to be using that line, so if you come from anywhere other than Glasgow Queen Street or somewhere on that route you will have to change trains. If you come from anywhere north of Edinburgh that means going to Haymarket and doubling back. And (according to Traveline) it's 7 minutes' walk from Edinburgh Park to TTH. That's a fair old distance if it's pouring rain or a blizzard.
Yes, there is a bus service, but unless you happen to live somewhere alongside those particular routes, you will have to change buses somewhere along the way, and pay double the fare of course. And (according to both Traveline and Lothian Buses web site) it is still at least 5 minutes' walk from the bus stops to TTH - still plenty of time to get soaked in bad weather.
I know lots of users of GRH (and NRH) who come into Edinburgh almost daily, by public transport, specifically to visit the National Records of Scotland and/or Scotland's People Centre, and who would find it very much more difficult to get to TTH.
It's rather ironic that the bus services that go nearest to Thomas Thomson House include some of the ones that drop you almost on the doorstep of GRH.