Because a Triton spends 10 Mana every time it applies a bandage, that takes away from it's use of FWW/PB. Plus, Najasaurus are completely Poison status immune, which is great in a spawn where a buttload of casters are on the pet, or in the Arachnid spawn. The Triton will be tankier against most mobs, but won't use FWW as often due to spending Mana on Healing.
There are pluses and minuses.
The minus is Najas is the fact that to come up to speed with the similarly speced Triton, you would need a 120 wrestling PS for 160 Million, also if you are on the middle of a nasty spellcaster spawn, and naja is ganked, naja can easily die since it cannot heal itself and you need to stay hidden usually. Problem is you will need also to PS naja's anatomy, med, and Parry to be able to sustain some damage. Thats another 50-150 million, depending on PS you use.
Triton does not have these problems as much : comes in with its own PS in wres and Magic resist, overcapped dex for quick action also like the naja.
It seems that it will not heal itself if not needed also. So no wasted healing attempts or mana if its health is good, such as with consume damage. It seems to perform amazingly well, even if not fully scrolled, and at 4 slots. Also it is able to TANK bosses like Neira, and clears the bosses summoned spawn with FzWW nicely.
I would think that Naja is likely a good option for the rich, that have 200-400 M to cash out for PS.
The two beasts speced similarly in my opinion are almost even, and imo Triton may be a little better exactly due to the healing, that comes handy to keep the pet alive in many situations. Plus it can actually tank due to its overcap wres and stay alive easier doing that. Mana did go down to 60 at the Neira champ, but it never went down to 0, and the little ugly thing kept dishing out punishment constantly to the soawn.
Having said all that I like Najas, and I wish they could have a Chiv/Ai option.