Well... that because first off it will need alot of work on it... on yr interior handling side and in the engine bay.... if its done by locally, then forget the peace your gona have after when your car is ready... But if you get it done by the company in professional way.. then it can work but won't be the same as it was... Plus its a bit costly unless your pocket is real deep!
I would simply say keep it like at it otherwise if you really wants a cruise control? sell the car and get a car that has all the features you need according to your specs!