Wikipedia is not reliable source. and especially in cases where no reference is given and sometimes i even checked the references given they were just of some survey works and not official. So always be careful while getting knowledge from wikipedia.

And about your question Why Mahmood only annexed Punjab.
It was nearer, and hence easy to control, could be made base camp for attacking northern, central and southern India.It was a fertile area so could fulfill needs of Aghanistan.

It was hard or impossible to annex and keep in empire other areas on India in his Empire because of remoteness and vastness of the Empire.

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"The only necessary thing for the triumph of evil is for good men to do nothing."
--Edmund Burke (1729-1797)