Socialism

As far as I know it's because both sides had pretty banal low-level and straightforward stated goals that were all "met" so there wasn't a clear "winner" and a "loser" in those strategic goals. It was really more of a 3 week skirmish than a full war. Vietnam obviously wanted to force China out of their country, and China said they wanted to bat Vietnam on the nose and force them to pull out of and not occupy Cambodia, or Laos or Thailand.

Which China left meaning Vietnamese succeeded in their strategic goals, and the Vietnamese diverted major resources and pulled out of Cambodia and didn't occupy Thailand and Laos meaning the Chinese succeeded. There weren't really any major strategic goals that were stated by either side that showed blatant failure; like China never said they intended to fully occupy Hanoi and create a Chinese puppet state and failed. Vietnam as far as I know never said they intended to continue occupying Cambodia or occupy Thailand and then failed to. So in a way they both got what they wanted and it was a status quo antebellum situation. Thus indecisive in the context of if it weren't 'indecisive' there would have been a winner or loser.

Thailand and Laos were under multi-factional civil wars whose royal governments were also US proxies; so the Vietnamese were also involved there (and involved with their local communist parties), prompting Sino-Soviet-split-related concerns with China since even though both China and USSR provided support to Vietnamese communists; the USSR became the dominant supporter and ally of Vietnam and continued to be. China also had an alliance with Cambodia dating before Khmer Rouge even; which was in part because Cambodia wanted assurance against the larger Vietnam and Thailand. The split in the Chinese Cultural Revolution era between the ultra-lefts and others had half of the CPC supporting the Prince and half of it supporting the Khmer Rouge against the prince. North Vietnam and Khmer Rouge provided support for each other for a while too. The politics were a mess. No idea what other involvements China had with Thailand and Laos other than Sino-Soviet fears.

People overstate the significance of Chinese casualties as meaning a loss when that's not how war works. Strategic objectives are all that matter. The losses (if you average the wildly disproportionate claims from all sides; impossible to actually know when you look at it) were more even than something like The Winter War between USSR-Finland; and though that war had the Soviets suffer disproportionate losses, it was still a complete strategic victory for the Soviets; they got everything they were after which had refused by Finland in previous requested land-swaps, namely gaining the Karelia buffer region.