I wonder why they would make it so the overlords of the entire foundation responsible for making world-altering decisions and who are frequent targets of assassination from hostile organizations would have more armor than mall cops?
Also everyone here seems to be arguing that, although ISD's primary role is to enforce the FLC, their secondary responsibilities involve combatting hostiles in some way, whether that be serving as backups in D-block or fighting off raids to protect non-combatives, and therefore that is why they need full armor.
While I understand where you are coming from, the fact that the conflict lies in the secondary objective means that you are admitting that the change does not hinder your primary objective. In addition, there are ways of protecting non-combatives without running directly at the threat and gunning it down, such as ensuring they reach a safe area and protecting that safe area, manning checkpoints, etc. which don't necessarily require a full set of armor, especially if you don't work alone.
As for the specific cases people have named where ISD would be most needed as direct combatives such as in a research wing hold, I will say that at least on the U.S side these are incredibly infrequent and also would not require the sole attention of ISD but rather every combative on site (barring maybe E-11 but lets be honest they'll probably join in too), so your assistance would hardly be crucial to Foundation's success on that end anyways.
Think of the problem from other perspectives as well - I'd argue one of gensec's primary or secondary duties is ensuring order within D-block, however they do not all spawn with handcuffs to arrest all wrongdoers? AO can't jail people even though among their duties is enforcing the FLC/COE? Techies are constantly planted right in the face of death to repair airlocks or bulkheads where hostiles or meat hungry monsters could be around any corner ready to show no mercy, yet they do not spawn with armor or weapons? A breach made it to D-block, but there was no E-11 on! Why don't cadets spawn with beams and cuffs? The point being, not every role needs to have EVERYTHING necessary to complete your job - that is the charm in the department bureaucracy of SCPRP - getting assistance from other people to complete an objective. Not every role needs to be able to do everything. These instances you name where ISD is "necessary," like in D-block when it's undermanned or to deal with a breach or a raid, I say this - you can't win em all. There are departments that are made to be good at those things, and ISD is not one of them.