One element which uniformed military services, the PHS Commissioned Corps, and the NOAA Commissioned Corps have in common which distinguishes them from civilian positions in government is that uniformed services put people who are members of them in situations in which the members of the corps are not permitted to quit once they have enrolled, until their term of service has expired.
Outside military or quasi-military service, the 13th Amendment ban on slavery and indentured servitude prohibits employment which cannot be terminated voluntarily at any time. There can be monetary penalties for quitting, but someone can't be compelled to do their job involuntarily.
But, just as the military needs to have soldiers who can't quit when they learn that they are being deployed to a high risk, potentially deadly assignment and could be ordered on suicidal missions for the betterment of the public, similar considerations apply to the PHS Commissioned Corps and the NOAA Commissioned Corps.
PHS Commissioned Corps personnel are deployed in emergencies to deal with often ill understood disease epidemics that can and do lead to the death of such personnel. NOAA Commissioned Corps members are sent into natural disasters that similarly present a high risk of death to people investigating and dealing with them. The NOAA Corps has also frequently served adjunct to military forces providing data for military purposes gathered at or near the front lines in what amounts to a military intelligence gathering function and could be impressed to do so in the future if it were necessary for them to do so. But, it isn't practicable to allow these highly trained personnel to simply back out of high risk situations so that the agency has to bargain in a highly unfavorable situation to obtain people with scarce skills to do the work on a market rate basis.
So, they have to be denied the right afforded to all other civilians to quit their jobs unilaterally, and this is most easily accommodated in the current legal environment by making them part of a uniformed service were precedents authorizing the inability to quit one's job notwithstanding the 13th Amendment are readily available and applicable.