The problem is that the original poem

A Christmas Carol

By Christina Rossetti

In the bleak midwinter, frosty wind made moan,

Earth stood hard as iron, water like a stone;

Snow had fallen, snow on snow, snow on snow,

In the bleak midwinter, long ago.

Our God, Heaven cannot hold Him, nor earth sustain;

Heaven and earth shall flee away when He comes to reign.

In the bleak midwinter a stable place sufficed

The Lord God Almighty, Jesus Christ.

Enough for Him, whom cherubim, worship night and day,

Breastful of milk, and a mangerful of hay;

Enough for Him, whom angels fall before,

The ox and ass and camel which adore.

Angels and archangels may have gathered there,

Cherubim and seraphim thronged the air;

But His mother only, in her maiden bliss,

Worshipped the beloved with a kiss.

What can I give Him, poor as I am?

If I were a shepherd, I would bring a lamb;

If I were a Wise Man, I would do my part;

Yet what I can I give Him: give my heart.

Copyright Credit: Christina Rossetti, “A Christmas Carol” from The Century Illustrated Monthly Magazine vol. 3 (1871-1872 Nov-Apr), ed. by J. G. Holland. New York: Scribner & Co: 1872. Public Domain.

has A) unusual words, construction and omission making it grammatically difficult to understand, and B) irregular word stress that no melody can fit every syllable of every verse into the same rhythm. Google Holst's In the Bleak Midwinter to see how different editors solve these in different ways, but Holst himself provided the to Ralph Van Williams for The English Hymnal in 1906: