Because they are trying to fit the Gradual back together from the pieces left over from the earthquake (1955) and the bombing (1969).

The Christus factus est is the Gradual for the Mass in Coena Domini -- there is no tract, of course. In the new order of Mass that text became the "Gospel Acclamation" and in the Ordo Cantus Missae it keeps the same chant, still called a "Gradual" . Similarly the "Responsorial Psalm" in the new Lectionary for Palm Sunday is verses from Ps 22/21, with response "Deus, Deus meus ..." , so the OCM kept the Tract (even though the verses are not the same ones, of course).

For Good Friday the situation is similar (except the shards and broken tiles of liturgy are rather more messily reconstructed). There is no Gradual for Feria VI in paraskeue, only two tracts. Neither survived, but the new orderers put down Psalm 31/30 for the "Responsorial Psalm" and reused Christus factus est for the Verse Before The Gospel as on the previous Sunday. Not finding a tractus for psalm 31, the OCM compilers chose the one from Spy Wednesday Domine, exaudi.

They did what the could, I guess.