[Avodah] Prophecy

[Cantor Wolberg via Avodah]

I quote the following (excerpted) from Oxford Jewish Thought - Essays by RabbiEli Brackman - Maimonides in Oxford: A commentary on the Oxford Manuscript of the Mishne Torah

" A known fact regarding Maimonides’ legal code of Mishneh Torah is the fact that it does not contain sources. 
Indeed, Maimonides received criticism for this and he desired to rewrite the work with all the sources but was unable 
to fulfil this ambition due to time constraints.”  ibidem: ",,,as he does not usually quote sources for the decisions in his legal code.”
I find it odd that a gadol like Rambam would omit sources to back his decisions, etc.

The other quote regarding prophets:
". In Mishneh Torah, Yesodei Hatorah (10:4), it discusses a difference between the substantiation of a prophet based on positive prophecy and negative predictions. 
The failure of the latter does not define him as a false prophet, while the failure of the former to materialise does define him as a false prophet.
The reason is because a negative prophecy can be annulled due to the fact that G-d is ‘slow to anger, abundant in kindness, and forgiving of evil. 
Thus, it is possible that they will repent and their sin will be forgiven, as in the case of the people of Nineveh, or that retribution will be held in abeyance, 
as in the case of Hezekiah.’ However a positive prophecy cannot be annulled and thus its failure to materialize can be a cause for him to be condemned a false prophet."

What I question is that according to the teaching if a prophet predicts a negative prophecy and it doesn’t come true, it can be annulled due to a compassionate God.  
On the other hand, Rambam states a positive prophecy cannot be annulled and thus its failure to materialize can be a cause for him to be condemned a false prophet.

So if God will forgive those who have committed various sins thus annulling a negative prophecy, why couldn’t the converse be possible — namely, God condemning those
who had been good and then turned to sin, thus annulling positive prophecy?