HBO has not released an official statement why they cancelled Crashbox. But there’s lots of data network execs look at when deciding whether to renew or cancel a TV series.
While HBO aren’t tied to the ratings like regular commercial channels, they have to air programs that will keep viewers subscribed.
You can't say that nobody watched Crashbox. But the show, though solidly educational, was aimed for grade-schoolers and probably drew meager ratings as it didn't appeal to the audience in general.
Ratings for initial airings don’t play as large a role as they do on commercial stations but they are still an indication of how popular a show is.
TV Tropes entry, Screwed by the Network, mentions a series of reasons TV shows are being cancelled.
TV shows that aren't making any money even with the network backing it, is one of the factors a program might head to its cancellation.
When HBO cancelled Vinyl — the serially clichéd rock drama, after just one season, the network already spent $30 million on the two-hour opener and $100 million on the entire first season.
And though they attached Terence Winter, Mick Jagger, and Martin Effing Scorsese to the series, it didn't survive through a second season. Not just because of its low viewership but it made HBO look bad.
Read the following articles sharing thoughts on why a network decides to abolish a television show:
TVTropes.org - Cancellation
TV.com - 6 Reasons Why Networks Are Trimming Orders Instead of Canceling Shows
Cracked.com - 5 Ridiculous Reasons Awesome TV Shows Were Canceled