Why Mawlid an Nabi is not declared as ‘Eid?
It is human nature that whatever bounty we get, in a few days we get used to it. Just see, eyes and ears are such a great bounty, but we don’t even think that these bounties have been given to us. We don't even think that thankfulness to the One who has given us these is Wajib (compulsory) on us. To revive the happiness of such a bounty, a reminder is needed and that reminder is the recurring of time.
As per the general custom, every day in a week and every month in a year recurs. Allah Most High has also acknowledged this. See, in the Eid of Qurbani, it is Fardh to not only remember all the incidents of Hadhrat Ibrahim (May peace be upon him), but to re-perform them. Hadhrat Ismail's (May peace be upon him) getting a new life has been declared as Eid for Muslims. Even the briefest reflection will show that on the face of it, Haj and Eid Ul Azha are nothing more than a repetition of this whole incident. When the second life of Hadhrat Ismail (May peace be upon him) is an Eid, then how much happiness should the Ummah feel on the birth of the Holy Prophet (Sallallahu alaihi wa sallam), who is the Pride of the Universe?
If it is said that instead of Eid Ul Azha, why Mawlid an Nabi was not ordained as Eid, then the answer to it is that the Prophets are more than 1 lakh and for every Prophet, there must be some incident, which caused them happiness, for e.g. for Hadhrat Adam (May peace be upon him), his birth, for Hadhrat Moosa (May peace be upon him), deliverance from Pharaoh, for Hadhrat Ibrahim (May peace be upon him), safety from the fire of Nimrod. From all these, only the incident of Hadhrat Ismail (May peace be upon him), who is the ancestor of our Prophet (Sallallahu alaihi wa sallam), has been ordained as Eid for Muslims. This shows the aim here was to give the Muslims an example that if they declare the birth of their Prophet as Eid, they should not be accused and no wonder that it is to test the Iman of the Ummah.
It is very clearly said that the one who does not love the Holy Prophet (Sallallahu alaihi wa sallam) more than one's life and wealth does not have any Iman. Then it is also imperative in love that one feels happiness with the presence and excellence of one's beloved and who does not feel happy has no relation with love.
The secret of not declaring Mawlid an Nabi as Eid seems to be that if it was ordained as Eid, then every person would follow the customs of Eid the way people do Haj formally and it would have been difficult to differentiate between the lovers and non-lovers. Now if someone says that love is there in my heart, we would ask what is stopping it from coming out. If they say that fixing a time/date, etc. is a Bida'a, then we would say a Bida'a is a thing, which has no precedent in Shariah. Here the precedent is present, Eid Ul Azha.
If someone wants to talk, there are great possibilities for it and it is not necessary for us to reply to every silly thing. We only need to present the guidelines of the Shariah that nothing stops the true lovers.
[Maqasid Ul Islam, by Hadhrat Shaykh Ul Islam Imam Muhammad Anwarullah Farooqui, volume 11]