Because it is the first introductory verse opening the MBh:
01,001.000a nārāyaṇaṃ namaskṛtya naraṃ caiva narottamam
01,001.000c devīṃ sarasvatīṃ caiva tato jayam udīrayet
on which, see 
Sylvain Lévi, 'Tato jayam udīrayet', Commemorative essays presented to Sir Ramkrishna Gopal Bhandarkar, ed. by S. K. Belvakar, Poona: BORI, 1917, p. 99-106; Translated by L. G. Khare. Annals of the Bhandarkar Oriental Research Institute 1, no. 1 (1918–19): 13–20 
and John Brockington, The Sanskrit epics, pp. 19 and 21

Le 1 févr. 2020 à 23:24, Harry Spier via INDOLOGY <indology@list.indology.info> a écrit :

Dear list members,
While doing a search of GRETIL  I noticed that the following verse is a Maṅgala verse in six different purāṇas.  As far as I could see the rest of the  Maṅgalam was different in each purāṇa.  Do any list members know why this verse should be so popular as a maṅgala verse in purāṇas.

oṃ nārāyaṇaṃ namaskṛtya naraṃ caiva narottamam  /
devīṃ sarasvatīṃ caiva tato jayamudīrayet  //

is a maṅgala verse in these purāṇas:
garuḍa-purāṇaṃ
kūrma-purāṇam
mārkaṇḍeyapurāṇam
viṣṇu-purāṇaṃ
vāmana-purāṇam
bhaviṣya-purāṇam

Thanks,
Harry Spier

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Christophe Vielle
Louvain-la-Neuve