Because toyota hilux is the rolls royce of insurgent vehicles. They're very simple vehicles mechanically, cheap, and easily obtainable. Isis had representatives in various middle eastern countries goin to dealerships/auctions and buying them right off the lots.
A youtuber called "feature history" did a video a year ago on "Toyota Wars". He gives some context on how groups came to use what would become known as technicals.
@ title IRA sympathizers in the US would buy guns on behalf of the IRA members in Ireland and have them illegally imported into the country. The most famous person to do this was George Harrison. A former IRA member who resided in the US.
They used Enfields, M1 Garands, and M1 Carbine.