This is purely an academic question, and one the answer to which I am really neutral about. It is put forth for consideration merely put into clearer relief a common notion regarding psalmody versus hymnody in the mass.
So: it is said that the hymn is a 'closed' form, sung for its own sake, which rightly demands to be sung in its entirety. Ergo, it is not appropriate at mass because, giving it its due, it may interupt the liturgical action by taking too long. In this it is contrasted with proper psalmody which is said not to be 'closed' and can be cut off whenever the liturgical action during which it is sung is completed. The (purely academic) question: why is it more appropriate to cut a psalm short of its completion than a hymn? Why is the psalm itself not equally 'closed' and needing to be sung in its entirety? Why is cutting a psalm short less savagery than cutting a hymn short? Well, of course we know that from early times introits and other psalms were cut short upon signal from the celebrant that it was time to stop. This, in itself, though, does not answer the question being posed here. Of course, too, we know that all to often hymns are savaged by being cut short, though most of us on this forum know that this is literary barbarism, the result of a purely time-conscious attitude to liturgy.
All things being equal, then, the question posed is 'why is a hymn a "closed" form and a psalm isn't - why is it not equally wrong to reduce the psalm to a few time-conscious verses'?

(And, as sort of an addendum, the concept that the psalm can be cut short as needed to cover liturgical action rather implies that its content is not too important or closely paid attention to... another ought-to-be-embarassing attitude towards the music [whether proper psalmody or hymns] which 'covers' processional activity at entrance, offertory and communion, as opposed to the meditative responsorial chant of the readings.)