Because they don’t know how to sim - or rather, how to account for discrepancy between the sim(bait) and actual play.
If you actually bothered to spreadsheet the data and look at your own logs from when S2 was current, you’d know that 2pc was worth ~2% on AoE and ~6% on single-target, overall, and the 4pc cooldown reduction was rather insignificant (approx. 2 extra casts in a 5 minute fight).
Compared to this, current S3 2pc is roughly worth ~3% on AoE and about ~5% on single target, while 4pc dire beast ability extension (beast cleave and kill command) is actually worth no less than ~7% in both AoE and single target scenarios.
S3 also has a crucial “but” baked into it.
S2 bonus is completely passive, so less competent hunters see it as competitive, particularly when looking at their own logs. Meanwhile S3 bonus actually scales with how well you can play, because amount of kill commands per minute varies immensely from person to person and depends on how you put your gear together. Better hunters get more kill commands out, and therefore get even higher numbers out, and adjust what to use when better, increasing their output even further.
To give you an actual number, in a recent PuG on Larodar, I’ve seen 65 kill command casts from the worst hunter, and 105 from the best. S2 bonus is completely flat and doesn’t account for how well you play at all. And it’s not like BM is just spamming one button, you have 2 cd-resetting or shortening abilities and a bunch of cooldowns which you shouldn’t neglect.
I think that the S2 supporters simply don’t like M+/AoE playstyle and resent the shift from “somewhat good single-target spec” to “overall good spec that shines on AoE”. That, or they ONLY raid. Sadly, we can’t have different season/tier bonus for raids and m+, so S3 trumps S2 as better overall.
