[klodolph]

Given that the NES had NMI and the 2600 didn't, I think that NMI probably should have been wired to horizontal blank to begin with. I'm sure that there is some reason why this wasn't done, but I can't understand why.

[jdsully]

That would have wasted precious cycles for games that didn’t need it. The ‘n’ in NMI stands for non-maskable.

[bonzini]

You can always mask it (enable/disable) at the place that triggers the NMI.

[jdsully]

Adding a a gate with jellybean logic was a noticeable expense in the 80s and would still be today.