Because my overnight attire when commuting never included business casual attire. I packed to be away from home for two consecutive three-day trips and didn't have room to spare. Also, wearing my uniform kept it out of my rollaboard, saving even more precious space.
...and no, I didn't wear the same uniform for six days straight. I kept one uniform at the dry cleaners at my base, one at the dry cleaners at home, and wore the third. In the middle of my three days (or when at home), I'd run the cleaners, swap my dirty uniform for a freshly cleaned one and be on my way without having to worry about their turnaround times.
Also, you don't have to be in uniform to take advantage of the fluid exception, however, if you are selected for additional screening, you must remain under the control of a TSA agent between the KCM terminal and security checkpoint...in other words, someone has to escort you to the screening checkpoint and hand you off to an agent there. If they are unable do to staffing (or whatever) only then are you screwed. I never took that risk while commuting since time spent in my base was pretty short and I didn't have time to replace my fluids.