... i feel like a lot of the reasoning behind Steem blokchain switching to 50/50 split in the upcoming HF21 is a commentary about Vote buying.

50/50 is not about vote-buying, nor is the curve. It's about reducing spam and abuse. I have seen many who felt the curve was to reduce bid bot use, but in fact, it will likely do the opposite.

50/50's main purpose is to encourage organic voting over just throwing votes on spam to get rewarded. Free downvotes paired 50/50 is what helps enforce this.

It also levels the playing field so curators feel rewarded and it is a more valuable activity.

In general, authors are up in arms and feel this is an attack on them but in the end, if it works well it should result in similar or better rewards providing behavior changes and there is enough flagging to reduce the rewards on garbage.

It will take time for these effects to be noticed.

Why are tribes using it?

Because Steem will, it is less friction if you match them together. Anything less than 50/50 (i.e 60/40) is likely not enough of a difference to see the behavior changes.

I can't speak for all tribes and their reasoning, I can only guess and tell you mine.

Some believe in 50/50, some are just using it because Steem will go in that direction, I suspect some are doing just because PAL did.