Why not a more conventional republican then? I don't think Trump has said he would repeal the ACA. There has been vague talk of changing it, but no evidence that it won't come out more populist than it is now.
Edit: This "Because I like fiscally conservative policies" answer is one I get a lot in these discussions... but it doesn't create a working model, because as far as I can tell, Trump is not a particularly fiscally conservative Republican.
No conventional republican was selected simply because of the way the GOP selects its candidates. With 30 people fighting each other, the votes were just too split up to allow anyone to get a nice majority.
Trump wasn't a particularly fiscally conservative Republican, but he was the only representative of the most fiscally conservative party on the ticket.
1. Potential for a 20% reduction in corporate income tax, a huge boon for small business owners.
2. Repeal Obamacare and it's obscene costs to small business.