Newt Scamander has researched a lot on magical creatures and yet if you compare his book Fantastic Beasts and Where to Find Them with Darwin's books, it'd look nothing.

Why did Newt Scamander write such a simple book which doesn't even have those details which Newt Scamander knew in Fantastic Beasts movie series?

The version we see is written for Muggles.

The two versions of Fantastic Beasts and Where to Find Them available for purchase are both written for Muggles.

“This edition, however, has a loftier purpose than the instruction of the wizarding community. For the first time in the history of the noble publishing house of Obscurus, one of its titles is to be made available to Muggles.”
_{^{- Fantastic Beasts and Where to Find Them}}

They’re for Muggles to read while still having the Muggles believe that the creatures described in it are fictional, so presumably the Muggle editions would be less detailed than the editions sold exclusively to wizards and never seen by Muggles.

“I would like to take this opportunity to reassure Muggle purchasers that the amusing creatures described hereafter are fictional and cannot hurt you. To wizards, I say merely: Draco dormiens nunquam titillandus.”
_{^{- Fantastic Beasts and Where to Find Them}}

Since Muggles are still supposed to not believe in the creatures written about, it’s very likely certain details would be left out, and the descriptions wouldn’t be as in-depth as in the editions for wizards. It’d be wise to leave out any ways of catching these creatures or particularly specific locations where any of them are found, lest any curious Muggles try to find them, leading to violations of Clause 73 of the International Code of Wizarding Secrecy.

“In 1750, Clause 73 was inserted in the International Code of Wizarding Secrecy, to which wizard ministries worldwide conform today:

Each wizarding governing body will be responsible for the concealment, care, and control of all magical beasts, beings, and spirits dwelling within its territory’s borders. Should any such creature cause harm to, or draw the notice of, the Muggle community, that nation’s wizarding governing body will be subject to discipline by the International Confederation of Wizards.”
_{^{- Fantastic Beasts and Where to Find Them}}

It’s logical that the version of this wizarding book that’s made available to Muggles would include less information than the wizards’ editions, since giving Muggles too much knowledge about magical creatures would be a danger to magic remaining secret. As Valorum mentions in the comments, the copies of Fantastic Beasts and Where to Find Them seen at Newt’s book launch are at least double the size of the Muggle version.

The version seen at Newt’s book launch is also clearly shown to be much larger than the Muggle editions in the Flourish and Blotts advertisement for it.

Therefore, it seems clear that the versions of Fantastic Beasts and Where to Find Them that’s sold exclusively to wizards contains more information than the Muggle editions.