To save on server calculations probably. Rather than checking every time 'which shield is stronger, for this person' across the whole party, it just overwrites it every time. Alterantively, it prioritises SGE shields because SGEis the dev's favorite child atmgets something when their shield is broken (addersting) and SCH doesn't, so it's 'more of a loss' for the SGE shield to be wasted than the SCH one, even though the SCH one cost a Deployment Tactics CD
yeh idk either, and considering how many shields we have access to now that DO stack (aka, all of them except Galvanise/Diagnosis/Prognosis) it's a wonder why they keep those ones exclusive. If they're worried about 'oh but then mit checks would be too easy if you had two shields on everything' then that just shows there's an issue with how they 'challenge healers', by making everything too mit-check heavy