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I assume thats the reason. As long as theres no interaction on a post, it can be deleted but as soon as someone interacts with the post, its permanent.

This is an interesting question. In Blockchain everything is decentralized. Anyone has access to view anything. We will be able to impose encryption but most of the things are kept open for everyone to view. There are both advantages and disadvantages in this.

The main advantage is that nothing can be hidden from the world. If we need a piece of information, it will be somewhere out there. We can always lookup. Due to this fact we will not be able to delete any transaction or broadcast in Blockchain. It becomes permanently available. This can be said as a disadvantage in a way but the weightage for it to be an advantage is more. That is why it takes a lable decentralized.

We will be able to hide a particular post or comment from visibility by adding a commit in top of the previous commit. If someone queries in the back end the previous version of the content will still be available but only the latest commit or broadcast will be visible in the UI.

It is indeed because of the rewards. Once somebody has voted on it he is earning curation rewards with it. It would be unfair to "steal" the other users their rewards for voting :)

Also note that there is no real deleting on a blockchain, everything is public and permanent once you post it, the only difference is if it is displayed on frontends like steemit.

This option could come in the next hardfork. some announcements are spreadly out already so it will be coming laters this year.

the form of permanent and no developers for changes on the website and application.

you can edit the post on steemit , title and tags as you are not able to delete a post , the previous post will still on blockchain but the viewers are not able to see it .

It's just the way the Steem blockchain works. A block builds upon the next block, and all the info is stored in all the blocks. That means that nothing can really be deleted from the blockchain unless you delete it before the next block hits, meaning you have 3 seconds.

That's not to say that things don't depend on the particular explorer you are using. It's possible that something can be developed to reveal 'versions' of former posts. Although things can't be deleted, it may be possible to mask with a separate version, or something... i'm sure they'll improve on this one day.