Why didn't the Church launch an "ecumenical movement" of dialogue with the Montanists and Marcionites?

Question

Why did they have to be so rigid?

Answer 1

The beliefs of Montanism contrasted with orthodox Christianity in the following ways:

1. The belief that the prophecies of the Montanists superseded and fulfilled the doctrines proclaimed by the Apostles.
2. The view that Christians who fell from grace could not be redeemed, also in contrast to the orthodox Christian view that contrition could lead to a sinner's restoration to the church.
3. The prophets of Montanism did not speak as messengers of God: "Thus saith the Lord," but rather described themselves as possessed by God, and spoke in his person. "I am the Father, the Word, and the Paraclete," said Montanus (Didymus, De Trinitate, III, xli); This possession by a spirit, which spoke while the prophet was incapable of resisting, is described by the spirit of Montanus: "Behold the man is like a lyre, and I dart like the plectrum. The man sleeps, and I am awake" (Epiphanius, "Panarion", xlviii, 4).
4. A stronger emphasis on the avoidance of sin and church discipline than in orthodox Christianity. They emphasized chastity, including forbidding remarriage.

Marcion rejected the entire Hebrew Bible, and declared that the God of the Hebrew Bible was a lesser demiurge, who had created the earth, but was (de facto) the source of evil.

Marcionites hold maltheistic views of the god of the Hebrew Bible (mockingly known to them as Yaltabaoth), that he was inconsistent, jealous, wrathful and genocidal, and that the material world he created is defective, a place of suffering; the god who made such a world is a bungling or malicious demiurge.

There is NO WAY to reconcile these beliefs with Christianity.

Answer 2

A very old conflict of values that's still with us...

Hold to pure truth (As a person/group sees it) even if that means conflict, disharmony and fragmentation of society.
Or...
Opt for harmony, and peaceful co-existence, even if that means (as a person/group sees it) allowing compromise and corruption of a truth to go unchallenged.

For example the Anglican communion is at present under much strain over the issue of homosexuality, not least since different regions are even divided on the relative priority of the two options above.

When does "ecumenical" slide over to "heretical"?
The Pope has had problems with that, attempting to define the relationship between Catholicism and Islam.