Only the Author, of course, knows why the bible doesn't mention certain things such as dinosaurs.   Science, on the other hand, however, cannot address what the Bible does address.  Those are questions of morals, values if life, and the meaning of life.  Science can be used to support some truths of the Bible, but most of the Bible is above the level of science.  The question about Dinosaurs does present an opportunity to think about some interesting possibilities;

A few possible and quite reasonable, but non scientific answers to the Dinosaur question could be:

1) Possibly, dinosaurs just weren't important in what God wanted to teach us.  After all dinosaurs did die out.  There certainly are enough bones laying around that God would know we would find them and enjoy figuring out what they were.

2) There are millions of species of living things, and the Bible would be a very large and awkward book to handle if all species were mentioned in it.  Can you imagine a book which described the creation of millions of species of life?

3) Possibly, God wanted us to think about how He created life in detail and appreciate the designs.   The details of life are incredibly fascinating.  Every cell in you body, for example, contains thousands of molecular motors!  The DNA in your body laid end to end could easily stretch to the sun and back!  The DNA is mostly coiled up inside the nucleus of your cells with about 5 feet of it coiled into a volume much smaller than the head of a pin.  It still controls from that tight ball without leaving the nucleus, the structure of all the proteins in your body!  Dinosaurs are trivial in their meaning compared to these facts.  

The Bible does not go into detail on scientific matters.  There is scientific support for the Bible, but it still is a book of faith and not a science book. 

God could have created life as we know it in many ways.  Even micro evolution (a fact in terms only of small changes to allow life to adapt to its environment) is reasonable as something that God gave to life along with other freedoms.   Keep in mind, also, that time for God is not the same as time for man or even time in the universe. 

Einstein theorized and numerous physics experiments have proven that time is not even absolute in this universe.   The origin of even time in this universe is a mystery science. 

The seven days for God, and the time, for instance, that dinosaurs may have been around (as perceived by God) may have been very short.  Dinosaurs may or may not have been required for life to end up has it is now.  God could have made things end up as they are now without the Dinosaurs if He chose.  In any case, how he did create life in detail is also not discussed in the Bible.  Maybe that is a puzzle that we aren't intended to know in detail.

The Bible is not a science book and does not contain any mathematics and does not discuss the laws of physics.  Most people probably would not have been impressed if it did.  The Bible contains much more important truths than the mere history of Dinosaurs.

Maybe the details of the incredible complexity of millions of forms of life is just something that is meant to be a wondrous mystery.  Maybe our study of it is one of His visual aids.

In general, science is much more limited that you probably believe.  Science only explains the relationships of things in the world and the universe.  It has nothing to say about where the laws of physic and chemistry come from, and those laws are what controls what science can address.  Science will not ever be able to say whether or not God exists.   That is a matter of faith, as is much of science, in fact, to some degree.