WHY can't Bridge sort images by date modified and STAY that way every time I come back?
By default it does stay at the latest selected sort order so this could mean it is due to your install.
First of all try to reset the preferences for Bridge, hold down option key while restarting Bridge and choose reset preferences. This sets it all to default. First try if the problem is solved then set prefs again to your own custom wishes.
Also check and repair permissions for the OSX itself (Apple has disk utility in the utility folder for this job, and there are other 3th party applications)
And be sure to have the latest update for Bridge. (Bridge CS5 should be 18.104.22.168)
In addition to the method Tai Lao pointed you already to there is a third option, use right click mouse button in content window and the pop up menu has also the sort option at the bottom of the row