The most obvious reason is that some people don't put the time and effort into showing why they should be granted parole.
In our free book, Understanding the Texas Parole Process, we explain how the parole process works, and also important statistics like the number of voting members of the Texas Board of Pardons and Parole and the number of applications for parole they review each year. From those numbers we can see that each application for parole and the file associated with that application can only get only a few minutes review. The numbers don't lie.
The majority of any parole file will be the paperwork from the criminal proceeding which sent the offender to prison as well as the documents from the prison itself. None of those are designed to cast the applicant in a favorable light. Instead, in many cases they are negative or at best, neutral.
That is where the Parole Packet comes into play. It is the chance for the offender to present any information which is favorable or which will cast them in the most positive light to the board member who actually votes.
What goes into a parole packet? That varies by individual and is covered in detail, along with examples and forms, in our book How to Prepare a Texas Parole Packet which is available in both digital format or as a digital and printed bundle. The printed book can be sent to the offender inside the prison system while the digital version can be used by family members "on the outside" who can assist in preparing the packet. While these books aren't free, they are substantially less expensive than hiring even a non-attorney parole packet preparation service (and, again, we recommend either preparing the packet yourself or using a parole attorney for reasons set forth on this website in various articles).
However, the actual preparation of the parole packet is the last step in a plan that has been carefully designed to maximize the positive aspects of the offender. This includes methods for the offender to educate themselves while in prison as well as obtaining certificates proving that they have not wasted their time while incarcerated but have, instead, taken steps to better themselves.
Anyone can make parole, a certain number will be granted each time and a certain number will be denied. The difference between those who do and do not get released can often be as little as just putting forth the extra effort to present themselves well in the parole packet.
The printed copy of the book also makes a nice present for an offender since it not only gives them something to read, but also gives them something to do with their time on the inside, which often seems to drag during the day.