The reason for it is quite simple. It is not allowed to publish any cell phone numbers with any information concerning it. This means that even publishing a list with cellular phone numbers with no other information can be acknowledge as a felony. But don't worry there are indeed regulations which allow certain companies to deal with cellular phone numbers, so that they offer a service of looking up this numbers. These companies have to abide by very strict rules and are subject to many controls regarding the safety of kept data in order to protect US citizens privacy in the best possible way.
So if you currently have the phone number, you would like to look-up, you will have to find a reverse cell phone look-up company. If you have never looked up any cell phones before, you may need an advice on choosing the best one. My personal recommendation is to click one of the banners on my website and use this service. I personally have checked at least 5 different website offering these services and after all need to admin to this one is the most efficient and worth its not big price. So when you open a window with this website you just enter the phone number in the text box placed in the middle of this website. Then just click "Search" or hit enter, whatever are your personal preferences. After a few seconds of waiting, the website should inform you whether the record has or hasn't been found. If they have it, you will be asked to pay a small fee to obtain the access to full details regarding the given phone number. The fees vary depending on the provider, but an average price for a single look-up is around $9.99. This may seem high to you, but the amount of information retrieved should persuade you that it's worth the money.
The information you can possibly get is for example, name and last name, current as well as previous addresses, household information, phone carrier, date of setting up the phone number, precise satellite imagery, other phone numbers of this person.