They didn't attack Obito in waves because they'd already clearly witnessed that it was pointless to attack him at all. That's why one of them ordered the others to grab Rin's corpse, they were clearly planning to retreat with it rather than keep fighting him.
That fight is one of my favorite in the entire manga. It's wonderfully dark and brutal, and really drives home just how fucked up the ninja world truly is. You don't normally get to see it that starkly what with most of the screentime focusing on the charismatic, idealistic stereotypically shonen hero that Naruto is. Seriously this manga could have easily been a super-depressing and dark fantasy series without changing the setting at all. Just imagine if someone like Kentaro Miura had written the series instead.