Universal quantification is restricted by conditional.
'All things $Q$ are $P$', 'All things have property $P$ if they have property $Q$' ... $\forall x~(Q(x)\to P(x))$
Notice how $\forall x~(Q(x)\land P(x))$ claims all things have both properties, which is _not_ what we wish to say.
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Existential quantification is restricted by conjunction.
'Some things $Q$ are $P$', 'Some things with property $Q$ also have property $P$' ... $\exists x~(Q(x)\land P(x))$
Notice how $\exists x~(Q(x)\to P(x))$ may be satisfied if there is nothing with both properties as long as there is something without property $Q$ or with property $P$. (A conditional is true if the _antecedant is false_ or the _consequent true_.) So that is clearly _not_ what we wish to say.