There are a few reasons to shelve it in favor of a new one. The absolute biggest of those is that the current rover is already WAY past its projected fail date. The last year or two has been completely bonus information. The upside of that is, bonus information from mars; the downside is that the projected budget for the rover for the last year or two wasn't ever factored in. NASA has been pulling funding from other projects to keep the rover afloat since it was deemed more important than some other areas. But, you can only keep robbing Peter to pay Paul for so long. At some point, and it looks like that point is now.. they had to stop funding on it completely and put funding back into areas they'd been neglecting, especially for something they aren't sure will stay in working order past the first day the new budget goes into effect. Its pretty much counted as one of the signs that a god does exist, that the rover has lasted this long.