How Flower-class corvettes mixed reservists and wartime crews

Why did Britain not put petty officers or NCOs in command of Flower-class corvettes instead of rushed officer candidates?

It relied on experienced reserve officers and senior ratings as the core of crews that also contained wartime officers and sailors.

Flower-class corvettes used a mixed professional core instead of relying entirely on rushed wartime officers. Experienced reserve officers supplied command knowledge that did not depend on a newly created wartime commission. Chief and petty officers added seamanship and practical authority, although not every senior rating already possessed specialized anti-submarine training. Wartime-only junior officers and sailors could then learn around that professional core while the Navy expanded escort forces rapidly.