Why didn’t Boeing ramp 737NG production back up in response to the 737 MAX groundings?
There are several reasons why.

First, it takes an awfully long time to make that kind of switch. While one can switch an assembly line for a product, that works rather differently for (say) transistor radios and Boeing 737s. The assembly line for an airliner winds back very, very far, and nothing happens or can happen immediately, or even very fast.

That would have made it very difficult, even if Boeing wanted to do that.

And here's a second reason: they didn't want to. They had orders for the MAX. If they were late in delivering those aircraft, they could be subject to financial penalties. Boeing agreed to deliver the aircraft, so had to build them. At the same time, they weren't facing overwhelming demand for other 737s that could possibly have made it worthwhile to do it in the circumstances.

A third reason: Boeing had every expectation that the aircraft would soon be allowed to fly again, and continued to operate under that expectation (for much longer than many observers shared the same expectations, it has to be said). 

A fourth reason: it would have been bad for business to do that. Doing what you suggest would have sent out the most appalling signals, i.e. that the company had lost faith in one of its most important products. In a way, they simply had to hope that things would turn their way sooner rather than later, and the option of turning away from that would have had tremendous costs. (You might argue that it was a gamble that didn't pay off; I think that would be fair enough to say.)

And finally, I think that Boeing's slow responses at many points in this story look like the behaviour of a company in shock. On multiple occasions, the company has found itself struggling to keep up with and make sense of events, even if they been events of their own making. Quite possibly the company's decision not to do as you suggest, or at least not to have done it much sooner, will turn out to have been a very bad one, but unfortunately, it will also have been only one in a long chain of them. 

By the time I make several bad decisions in a row in a crisis, it's pretty much guaranteed that the next one I make will make everything even worse. Organisations also fall victim to that pattern.