Taking into account how all initially started it's normal that they didn't tell anyone at the beginning. They didn't know what to think about it themselves. Their conversation in hospital, when Phoebe was in labour, in episode The One Hundreth can be seen as a beginning of a real relationship.
Later they still they refrain to tell anyone. They explain their reasons to Joey when he finds out two episodes later, in The One With All the Kips:
Chandler: The reason we didn't tell anyone was because we didn't want
to make a big deal out of it.
Joey: But it is a big deal!! I have to tell someone!
(They both grab him and stop him.)
Chandler: No-no-no-no-no! You can't!
Monica: Please. Now, I know this is hard.
Joey: It is. It hurts.
Monica: We know. Please? Please?! We just don't want to deal with
telling everyone, okay? Just promise you won't tell
So they were worried about the reaction of the rest of the gang and they decided to keep their secret until they felt that they were completely ready to reveal it. Perhaps they waited too long and it kind of backfired on them later, but I won't spoil too much, since I don't know how far in Season 5 you are.