Time is definitely an issue, as is concern for Juliet's well-being. He appears to want to tell them (and Tybalt too) but says he has to wait for the right occasion to do so.

There are a couple of other reasons why Romeo keeps this secret. One is that he is sensitive and his friends will most definitely mock him (just listen to what Mercutio says when he is looking for Romeo after the party). Also, Mercutio tends to sexualize everything, and although sex (or the promise of it) is certainly a motivating factor for Romeo, he also wishes to frame his new relationship in terms of love. It would not be possible to flounce around wearing 'love's light wings' around his friends--Mercutio would bring him down to earth with a crash. He also says something that suggests he really can't trust them to understand (he says of Mercutio...he jests at scars that has never felt a wound).

So timing is definitely a factor, but also, perhaps, is the fragile condition of his heart. Perhaps he wishes to preserve the emotional and spiritual bliss he experiences with Juliet without having it marred by the crass commentary of his friends, at least temporarily.

After the night in which Mercutio and the others search for Romeo, who has scaled the orchard walls of the Capulets in order to see with love dazzled eyes she who "doth teach the torches to burn bright,"  Romeo visits Friar Laurence.  At the priest's cell, Romeo asks the friar to perform the marriage rites for Juliet and him.  Amazed, Friar Laurence exclaims,

Holy Saint Frances, what a change is here!

Is Rosaline, that thou didst love so dear,

So soon forsaken?  Young men's love then lies

Not truly in their hearts, but in their eyes.

Jesu Maria, what a deal of brine

Hath washed thy sallow cheeks for Rosaline!....

The sun not yet thy sighs from heaven clears,

Thy old groans ring yet in mine ancient ears.

Lo, here upon thy cheek the stain doth sit

Of an old tear that is not washed off yet....

Thou and these woes were all for Rosaline.

And art thou changed?  (2.3.68-77)

Since the priest has chided him and since the Capulets are the sworn enemies of the Montagues, Romeo may be more than a little reluctant to reveal his infatuation and increasing passion for Juliet.  Certainly, he does not want to broach the subject when he meets up with Mercutio, his cousin, who is engaged in a heated dialogue with Tybalt in the streets of Verona.  And, it is possible that Romeo considers the parting words of the priest, "Wisely and slow.  They stumble that run fast" (2.3.97)

One reason, at least to me, is that he never got a chance to tell them. When he enters in act 3 scene one, Tybalt and Mercutio were already in a heated battle of words. Romeo feels the need to get in between them to stop what was happening. He kinda does tell Tybalt why he doesn't want to fight him but Tybalt and the others don't understand his riddle. Don't forget, the only people that even knew that Romeo fell in love with Juliet and just got married are the Nurse and Friar Laurence.