Why is "Murder in the Cathedral" called a poetic play?

Because it's written in verse. You might hear it called a poetic play, or indeed, a "verse drama": all that means is that the dialogue is patterned into a verse form. If you looked at a script of - say - General Hospital, you'd find that you couldn't put the dialogue into a rhythm most of the time. The lines would be as long - or short - or contain as many syllables - as they liked, as fitted the moment.

Verse drama means that the lines all fit onto a metrical grid: all follow a verse pattern. Shakespeare's plays, for example, are partly written in iambic pentameter (de DUM de DUM de DUM de DUM de DUM) - and a proportion of the lines fit to that rhythmic grid. Eliot's verse line is more unusual, a longer, less regular line - but the plays are still written in verse. Hence why they are "poetic drama". Eliot had hoped to revive verse drama (largely unpopular in theatre since the Elizabethan age) - but it didn't catch on. Plays these days really aren't written in verse.

There's an excellent website linked below with more information about Eliot, and his ambitions to write drama in verse.

Hope it helps!