Two Words: Muslim Brotherhood
Because Assad is seen a source of stability in the country. One of the few reasons why we turned a blind eye to Syria until Hariri's assassination unraveled it all. Btw the Bush people preferred an Alawite general [not associated with the Assads] in power and I'm sure the Obama people do as well.
Syria is increasingly becoming more and more sectarian [Christians cheering government troops in Homs, Sunnis harassing Shi'ites in the Sayyeda Zainab district of Damascus] which doesn't bode well. Syria 2011-2012 has potential to turn into Iraq 2006.
Syria is a country where a "heretical" group, the Alawites, which is nominally closer to the Shiites and politically aligned with the Shiites of Iran, Iraq, and Lebanon, is lording over a mostly Sunni population (65% or so). But not any Sunni population, but the proud inheritors of ultra-Sunni symbols such the Umayyad dynasty, Saladin, and Ibn Taymiyya, all of whom are buried in Damascus. It's low-hanging fruit. Many of Assad's opponents are acutely aware of the hadith of Mohammad that the lands around Damascus will be the center of an Islamic revival.
Of course this is beginning to sound like Iraq in reverse with its ultra-Shi'ite symbols such as Karbala, Najaf and the Imam Musa Kazem + Imam Mohammad al-Taqi mausoleum in Baghdad.
Another thing to note, Syria, plays a role in another country with a deep sectarian divide that had a brutal civil war, Lebanon.
