Your reason is the more obvious one.

From: Rickard A. Parker <[log in to unmask]>
To: [log in to unmask]
Sent: Tuesday, September 13, 2011 7:38 PM
Subject: Re: Eliot vis-a-vis the mystic 3

I like the first two especially but another reason for the dropped "Om"
could be that he forgot about it.

    Rick Parker

On 9/12/2011 10:50 PM, Chokh Raj wrote:
> /some more thought/
> I was just exploring why Eliot chose to drop the initial "Om" from the
> closing incantation of TWL.
> Well, the reason could simply be prosodic -- the poet would be governed
> by his new requirements of rhythm.
> He would also quite obviously find the triple incantation "Shantih
> shantih shantih" quite apposite beside the preceding three-fold mantra
> "Datta. Dayadhvam. Damayata".
> He would, of course, uphold tradition without sounding too pedantic.
> Above all, Eliot has his own manner of borrowing material. He borrows in
> snatches -- suggestive, terse, parts standing for wholes.
> I'm not sure Eliot dropped "Om" (the logos of revealed truth) to suggest
> the poem had not reached a point of full illumination -- a view advanced
> by Cleo McNelly Kearns ('T.S. Eliot and Indic Traditions: A Study of
> Poetry and Belief', 1987). It could albeit suggest the wastelanders are
> not yet ready for the grace of mystic light. The poet, therefore, makes
> his prayer for peace in terms that might carry conviction with them.