Ibrahim ashtar had joined hands with zubayris. Why did he give his bayat (allegiance)?

According to history, after martyrdom of Mukhtar e saqafi, Ibrahim recieved two different letters from mus'ab bin zubair (The Ruler of Iraq) and Abdul malik bin marwan (The Ruler of Sham). He was offered from both side to join them and promised to get the ruling over a specific region. Ibrahim took the suggestion of his nearest and reliable persons. Their opinions were differing from each other. Some were suggesting to join Mus'ab and the others were suggesting to join Abdul Malik. Finally Ibrahin took decision by his own mind and heart. Actually he was much more inclined with Iraq because Hhe had grown up there and wanted to rule over there. Apart from it he had fought with Sham Army in a very important war named in history "War of the Khariz river" he defeated the Sham Army in commander ship of Ubaildullah ibn Ziyad and finally got victory over them in that significant war so he had killed a lot of sham soldiers therefore they had a grudge against him and they may harm him in the future so ultimately in that situation he decided to go with the sons of zubair because he had to join at least one of them to protect himself, his friends and family and there was no option to be neutral for a personality like him in that situation after fighting with enemies in many wars specially with Mukhtar and having a position of the Governer of Musal (Iraq).