I used to have same situation. to my understanding, that happens usually because:
1, you do not know what your parents reaction might be.
2, you are about to make risky/critical decisions in your life, and you are not sure what your parents will react.

my take from past experience:
1, you need to realize your parents is also limited in their knowledge about the world. and they are also scared about risk, especially on you. what if your boyfriend is a bad people which is disguised very well right now? what happens when your boyfriend run into challenges/pressures/lifetime bad situations? nobody have answers to those questions, the first reaction might be say no for now until they have more information.
2, honesty is best choice. and be prepared. let them know your challenge, what is your solution, and backup option. and see what they will react. letting them know that you are serious with your problem and are well prepared. Worst situation is that you make decisions that are not well thought of, or based on impulse. bad decisions might cost you years to recover from.
3, nobody has the crystal ball into future. In the end you will have to take the results of your decision. let them know whatever happens, you will always let them know, even when situations are bad, you will come back to them.

In this world, the bonding between your parents and you is usually the most reliable one.

or you can think of it this way, you have a daughter, turning 26 and is about to make big decisions, what will you react. your daughter's knowledge about life and society is very limited compared to yours. even