Because you can't have NI adhering to UK rules, and ROI adhering to European rules and then not have a hard border running between the two.
We will have different standards and tariffs. It becomes open to abuse. This is the main issue that Brexiteers can't get their heads around. If you have two separate markets, you will need checks and balances at either side.
Lets say we do what you propose though....here's an example of what will happen....
Lets say we have a tariff of 20% on cheese imports to protect our domestic cheese market (please don't now go into all details of things about cheese, I've used it as an example only)...but the EU is tariff free for Cheese. A large importer could in theory bring it to ROI, move it without any checks at the NI border. And then infiltrate it into the UK market without paying the 20% tariff and undermining our domestic Cheese industry - of which thousands of jobs rely on. Because of cheap imports. (It doesn't have to be cheese, it could literally be any product)
You could also use the same example about any food products that may be banned in the UK, but not in the EU. And of course this works in reverse, there may be food standards the UK now lower which may not meet EU standards.
So anyone who says that we should just be able to move products tariff and check free between the UK and EU (which we had in the SM), and that the EU are being difficult...can they spot the issue above? The reason we have tariffs is to protect domestic industry. The reason we have checks is to uphold our own health and safety standards.
By leaving the EU and negotiating our own trade deals, the EU were never going to give us the same access to their market as when we were in the Single Market, and why would they? We could undermine their industry and fill their market with products that don't meet their H&S standards.
I've heard a lot of stupid people say things like the EU are being awkward and difficult over the NI border, and by not giving us access to the SM. These people are deluded and/or stupid and/or ignorant. They don't deserve any time of day.