They failed to keep up. It's that simple.
I use Google Maps as my primary mapping service. Have done for years. Streetmap's data was better, but they stayed static for far too long.
I just visited Streetmap to see how they were doing, and they have a bigger mapping window now - but not big enough when you compare to Google Maps or OpenStreetMap. I was pleased to see that they did support grab-scrolling, but disappointed to see that scroll wheel zoom didn't work - it just moved the whole page.
Basically, they haven't kept up. Google's mapping data is good enough, and they just keep adding features. Their integration with their search is superb, they've added directions, street view, live traffic reporting...
Anecdote time: I don't drive but was on a trip with friends recently (to a distillery, so why drive?) and on the way back we hit a traffic jam. My phone alerted me that it was roadworks, and with some judicious scrolling and checking the live traffic overlay on my phone I managed to locate exactly where they were, which seemed to help us all to stay more sanguine about the experience.
Frankly, if I'd had the presence of mind to check my phone beforehand, Google Maps could probably have saved us some time by getting us a route that avoided those delays!
And I said that Streetmap's data was better, but that past tense is deliberate. It's missing some paths in local parks. Not new paths either, but ones decades old. Google was missing them a few years ago but is slowly adding them in. OSM has had those right for ages.
Finally, let's not mention the woeful search. Both Google and OSM could get me to a local park by name, Streetmap couldn't manage it no matter which option I picked. And it's 2017 - why do I have to pick a search option? Search them all, then show me a list!
I have fond memories of printing out an occasional Streetmap page back in the early 2000's. Before phones had mapping and internet connections, A Streetmap printout was more convenient than carrying an A-Z around, providing your journey was short. But they have more competition than just map books, and they seem to have failed to realise that.