I really want to know why second marriage or 4 marriages are allowed to men when this can be and in most cases is distressing for the first wives (varied culturally)
Rasoolallah was allowed 9 and from what I know this was for tableegh purpose, but why are all men allowed??
Also, neither Rasoolallah nor Imam Ali married another woman during the life of Bibi Khadija and Bibi Fatima. Why is that? Was it only because of the love they shared with each other?
Then what was the need to have several wives at the same time, after Bibis' shahadat? Was the love shared then with any one wife was not enough?
God forbid and forgive me if anything here is disrespectful to any personality because of the way I've explained my question
my intention was never to be disrespectful

Alaykum assalam
In fact, plural marriage is not allowed in Islam, except if the man is 100% sure that he is able to treat all the wives equally.
Otherwise it is forbidden.
First of all, let us agree that this order is from Allah SWT, not even from prophet peace be upon him and his family, and that Almighty is not questionable why he did so, and that he is wise, and knows what are the pros and cons of everything.
On the other hand, we might try to work out some benefits of the plural marriage, but we can not say that these are the real reasons for allowing it.
One of these reasons could be, the difference in female population compared to male population, which is very clear, and could be very easily touched. Hence, having one wife for each man will result in big number of single women, which is very dangerous situation, while if each man got married to at least two women, the number of single women will decrease dramatically.
We need to accept what Allah says.
If we did so, then there will not be any bad feelings, mainly if the first wife admitted that her husband loves her.
In fact, should encourage him to have another wife, in order to solve a social problem.