Why is the Bible so vague, meaning why can't most Christians or so called Christians understand the dispensation of Paul's message? The difference between the Gospel of the Kingdom and the Gospel of Grace.

I think that there are a number of obstacles that keep people from understanding the difference between dispensations. Let me give you at least 3 reasons why folks don't or won't understand the distinction.

1. Most Christians don't study the word of God. If the Bible is "vague" on some things that are important, those things can be cleared up by studying the Book. Folks who don't study the word of God have to rely on what others tell them it says. Their reliance is primarily on the teaching of folks who have gotten a Christian education at one of the large, recognized Christian colleges or universities. Many, if not all, of those schools teach that Dispensationalism (as taught by C.I. Scofield and Clarence Larken) is wrong. Guess what those folks, for the most part, are going to teach their people when they graduate? Same stuff they were taught in college.
2. In addition, one of the implications of teaching a difference between Paul's gospel of the grace of God and the gospel of the kingdom is the fear of being branded a heretic for recognizing a difference in the plan of salvation within different dispensations. The standard mantra among non-dispensationalists is: "Folks before Calvary looked forward to the cross just like we look back to the cross." I believe that there is an element of truth to that statement. Salvation is based entirely on the grace of God and the blood of Jesus Christ, but that is from God's perspective-God forgave folks before Calvary based on the blood that would be shed, but those folks, for the most part, had no clue about the death, burial, or resurrection of Jesus Christ! Those things are absolutely essential for salvation today.
3. Finally, there is a common fear among folks who don't study the word of God and have been saved for a number of years. They fear anything that they have never heard taught before. They think that if something is so, they would have heard about it long before you showed up. That, however, fails to recognize that most preaching and teaching today isn't balanced. It is primarily milk and not meat. There is nothing wrong with milk, especially for babies (1 Peter 2:2As newborn babes, desire the sincere milk of the word, that ye may grow thereby:
   See All...), but folks cannot grow in grace and knowledge of our Lord and Saviour Jesus Christ (2 Peter 3:18But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen.
   See All...) drinking only milk. They'll grow for a little while, and then that growth will stop. They need some meat in that diet too.

Let me say in conclusion that there is a frustration that most of us have experienced over the inability of others to see something in the word of God that we see. It's hard to be patient and show grace to people if they don't embrace the truth as soon as they are exposed to it. It is helpful to remember that the Lord Jesus Christ taught His disciples some things that they didn't grasp right away. In John 12:16These things understood not his disciples at the first: but when Jesus was glorified, then remembered they that these things were written of him, and that they had done these things unto him.
See All..., one of those situations is recorded: "These things understood not his disciples at the first: but when Jesus was glorified, then remembered they that these things were written of him, and that they had done these things unto him."