Despite the mythology that some people seem to believe, a president is nothing more than the nation's most successful politician at any given time. Like virtually all politicians, he will weigh the political risks verses benefits before taking a stand on most issues, including space exploration. If, as has happened to Bush, announcing a major change in space priorities will succeed in nothing more than getting jumped by both his opponents and even a big percentage of the space advocates, then why bother? It's not like his stand on space exploration will gain him that many votes in the population at large, so the potential benefits are poor compared to the risks. Ultimately, that's what matters to most politicians.