Why didn't the dinosaurs enter the Ark?

It needs to be clearly understood that many dinosaurs died in the global flood of Noah's day. Secondly, it is also highly likely that there were representative kinds of dinosaurs, most likely young specimens, taken aboard the ark. There is absolutely no reason to assume that they weren't taken aboard, as they would have been included in the 'two of every kind' that God brought to Noah, to preserve genetic diversity on the earth.

Also, it must be remembered that the climate of the earth underwent drastic change due to the processes associated with the flood, including the ice-age which followed it. As a part of this process, the dinosaurs which went off the ark most likely then became extinct. It is interesting to note in this connection, the widespread existence of 'monster' or 'dragon' accounts, commonly interpreted as legends among many of the people groups of the earth, including in China and some quite recent accounts from England.

So, to quite simply summarize the answer: Noah took dinosaurs on the ark. Secondly they became extinct afterwards, in the drastically altered post-flood environment.

They must have been on the ark probably as adolescents to compensate for their size.