why should we upgrade bigip version 9x to 11x

One of the reasons is v9 has reached end of life.

There are no more enhancements/improvements/major fixes introduced in v9 anymore.

• Lots of new features and enhancements.
• No-one will answer your questions on Devcentral about v9!!
• At least go to v10 (which you will have to do anyway on the way to v11).
• As hwidjaja says it's eol and you won't get any support.