May be, because as per Devi Bhagavatam,Hayagriva got a boon from Devi:
93-95. Hayagrîva said :-- “Obeisance to the Devî Mahâmaye! I bow down to Thee, the Creatrix, the Preserver, and the Destructrix of the universe! Skilled in shewing favour to Thy devotees! Giver of the devotee's desires! Obeisance to Thee! O Thou, the giver of liberation! O Thou! The auspicious one! I bow down to Thee. Thou art the cause of the five elements -- earth, water, fire, air, and Akasa! Thou art the cause of form, taste, smell, sound and touch. O Mahes’vari! the five jñânendriyas (organs of perception) eyes, ears, nose, tongue, and skin and the five organs of action Karmendriyas :-- hands, feet, speech, arms, and the organ of generation are all created by Thee.
Hayagrîva said: -- “O Mother of the universe! If it be that Thou art not willing at all to grant me immortality, then grant me this boon that my death may not occur from any other than from one who is horse-faced. Be merciful and grant me this boon that I desire.”
101-105. O highly fortunate one! “Go home and govern your kingdom at your ease; death won't occur to you from any other beings then from one who is horse-faced.” Thus granting the boon, the Devî vanished. Becoming very glad on getting this boon, Hayagrîva went to his residence. Since then the wicked Daitya is troubling very much all the Devas and Munis. There is none in the three worlds to kill him. So let Visvakarmâ take a horse's head and fix it on the headless body of Visnu. Then Bhagavân Hayagrîva will slay the vicious wicked Asura, for the good of the Devas”.
To kill him, Vishnu also had to take a 'Hayagriva' form !:
106-112. Sûta said :-- Thus speaking to the Devas, Bhagavatî S’arvânî remained silent. The Devas became very glad and spoke this to Visvakarmâ :-- “Kindly do this Deva work and fix Visnu's head. He will become Hayagrîva and kill the indomitable Dânava.” Sûta said :-- Hearing these words, Visvakarmâ quickly cut off with his axe, the head of a horse, brought it before the Devas and fixed it on the headless body of Visnu. By the grace of Mahâmâyâ, Bhagavân became horse-faced or Hayagrîva. Then, a few days after, Bhagavân. Hayagrîva killed that proud Dânava, the Deva's enemy, by sheer force. Any man, hearing this excellent anecdote, becomes freed, certainly of all sorts of difficulties. Hearing or reading Mahâmâyâ's glorious deeds, pure and sin destroying, gives all sorts of wealth.
Reference : http://www.sacred-texts.com/hin/db/bk01ch05.htm