There is really nothing much more to say than those Hummer sales were so low in 2009 that GM has decided to discontinue the line. With the rising cost of gas, and Hummers being far from the most fuel-efficient vehicles, one need not be a Jean Dixon to have predicted the tumultuous decline in sales.
The hopes that many held that Hummer would produce a more fuel-efficient model were dashed, and this was pretty predictable as well, since the 2009 release of the model was preceded by the summer of 2008 seeing some of the highest oil prices of the year. By that time, production of the 2009 model was well underway without accounting for fuel efficiency considerations. When the sales for Hummers in 2008 fell over 50% from its sales in 2007, it could only follow that the decline would continue without changing its fuel-efficiency factor.