Yep, this is the key point:
Jack, and Kieran, are not - to the best of my knowledge - directly employed by the park / Merlin. They effectively operate as contractors. This isn't too dissimilar to how Thorpe work with other companies, such as Unique Concepts. In short, by hiring them as contractors, Thorpe get their expertise, whilst the contractors (in Jack's case, himself) can get more money than if they worked solely for Thorpe, and are free to work for other companies too. If anything, Thorpe are the ones who lose out slightly, but that's the way they go about things.
Access will in part be restricted for H&S reasons. I think it's notable that during Jack's on-site construction updates, it is only him (and not him and Kieran) on-site. There's a reason why we rarely see such in-depth coverage of coaster construction. I expect that the signing off and training process that Jack had to go through to be allowed on site is a lot.
When Coasterstudios did their update, I noted that they were only ever on the viewing platform above the offices. So they likely went through a different H&S sign off to be allowed on site. I'm certain that their presence was pre-approved by Thorpe, and not Jack just having a jolly with mates and letting who he wanted on site.
As for why them and no one else. Ultimately, Coasterstudios is huge across the pond, so will get a little more enthusiast buzz created in America, for very little cost.
Now here's the thing, with the more Thorpe / UK centric sites. Who do you choose outside of Jack?
Do you choose DG95, who is clearly very passionate, but has a limited fanbase and who's videos are more rough the edges?
Do you choose Attraction Source, a large fansite who work with Merlin for paid extra events and produce higher quality videos?
What about ECC, with their broad reach?
Or CoasterForce; again somewhere with broad reach, one of the longest-running fansites about and who produce quality content?
And what about TPM? We're the only sole-Thorpe-focused fansite around now; our reach on social media is very big (and our Facebook, in particular, attracts people far beyond the usual enthusiast circles).
So yeah, how do you choose? Why should any of them get access?
Maybe it's just a case of asking - I don't know if anyone else has, but I know TPM hasn't. Ultimately, as awesome as it would be to go onto a coaster construction site, I don't know what TPM (or anyone else) can offer that's different to the content that's already been produced by someone else. So why should anyone else get access...