Why doesn't the Room of Requirement show up on the Marauder's map?

We don't know.

Hermione and Ron offer differing (but not necessarily contradictory) explanations for why it's not on the map. Hermione surmises that unplottability may be one of the charms that the room casts when it's being used by someone who doesn't want to be found whereas Ron's suggestion is that it may not be on the map because the Marauders didn't know about it.

Since we know that the map only shows rooms and passageways that were known to the Marauders (hence the Chamber of Secrets not showing up) there's a distinct possibility that they're both right; that it's not on the map because the Marauders didn't know about it, but that even if they had, they wouldn't have been able to add it to the map because it has its own cloaking spells.

‘The Room of Requirement!’ said Harry, smacking himself hard on the forehead with Advanced Potion-Making. Hermione and Ron stared at him. ‘That’s where he’s been sneaking off to! That’s where he’s doing … whatever he’s doing! And I bet that’s why he’s been disappearing off the map – come to think of it, I’ve never seen the Room of Requirement on there!’
‘Maybe the Marauders never knew the Room was there,’ said Ron.
‘I think it’ll be part of the magic of the Room,’ said Hermione. ‘If you need it to be unplottable, it will be.’
‘Dobby, have you managed to get in to have a look at what Malfoy’s doing?’ said Harry eagerly.

_{^{Harry Potter and the Half-Blood Prince}}

We do know that it doesn't show up at other times (if we presume that Harry has looked at the map other than when Malfoy is in it) suggesting that Hermione may be wrong that it's only unplottable when needed, more likely it's unplottable at all times.