Friend,
A good question indeed.
Observe all the DASAAVATAARAS.
Except Sri Krishna and Sri Rama the other AVATAARS of Vishnu , ended their Avatars immediately after the purpose for which the AVATAAR has come.
But Sree Rama was born as any other human, had childhood experiences, studied in Gurukulam and learnt all the arts that are necessary to a Kshatria and also Veda vedaangaas through Vashista and others and , married lived a life as any prince..Then after marriage , living after 12 years in Ayodhya HE ,RAMA was compelled by the word of the Father to go to Forests.Here the SITA was abducted by Ravana and rama after all sufferings FINALLY KILLED RAVANA (FOR WHAT PURPOSE THA AVATAR CAME).But , after KILLING RAVANA Sree Rama reined for 11000 yeras the Ayodhya kingdom and that period is still remembered as RAMA RAAJYAM...Then finally HE entered SARAYU RIVER and coimpleted HIS AVATAAR.So it is called PARI POORNA AVATAAR.
Similarly Sri Krishna took birth as any human, and was transfered to Yasoda side.He had shown so many balya leelaas.He grew with Gopalaas and Gopikaas and all were HIS BHAKTAAS .
Krishna too had married and killed the so many for which HE came to Earth, that is NARAKAASURA.
But after killing him , he too reigned Yadava Raajyam for many years and was with Pandavaas guiding and protecting them.
In Kurukshetra war Krishna is the HERO really.
Then after that pandavas did Rajasuyam and after that KRISHNA LEFT THE BODY.
This is also a POORNA AVATAARA.
This way THESE TWO RAMA AND KRISHNA AVATARAMS ARE POORNAAVATAARAMS.
.
-Anonymous