[ALV]

From the editorial aspect:

The difference between GMT, Newsday, Impact, WNA etc... and Newsnight is that Newsday, Impact, WNA are classified as general news bulletins -  that consists of 50% US/Europe/Asia news while you also get 50% of news from other parts of the world (during non-breaking news days) (at least that's what the intended plan is.) So it's not completely irrelevant to the global audience. They are intended for a general world audience who want a little more news from these regions, yet without going super-in-depth on a particular country that they lose context if they're not familiar with the story.

For programmes like Newsnight, it's like 80% domestic news with super in-depth ground level investigations, interviews and analysis on a handful of topics. It's not a news bulletin, it's a current affairs programme. The ident announcer pretty much sums up the scope of the programme:
"Now on BBC World News, hard hitting interviews and documentaries from around the world - Newsnight with stories making the news this week!"
(Yes it says "from around the world" but anyone who watched the WN repeats would know it's mostly UK stories..."

So putting Newsnight on weekends as a summarized 30 minute back-half pre-record makes sense, it's just one of the current affairs programme with an in-depth look on a certain part of the world that deem to be of a certain interest to global viewers. That's one of the purpose of the weekend pre-record blocks - providing an in-depth look on a certain place every week. 

However, if we would to air Newsnight every single weekday, in full 40 minutes every night, it would just overwhelm viewers. Not only is Newsnight NOT a news bulletin, but the weekday schedule on WN is not designed to make documentaries, current affairs programmes take up the majority of the schedule. The weekday schedule are designed to take up general news bulletins, business, sport instead.  

Yes, I do realize some documentaries like Our World and current affairs programmes like HARDtalk take up a bit of schedule on weekdays. But, those programmes usually air on the deadest slots in Europe, like late overnights and early morning. To me it seems like they're just there to give viewers a last chance to catch up these programmes before it goes off air, since there's no equivalent of "iPlayer" on WN.

(The current international VOD services like "BBC First" (for Asia), "BBC Player" (for Singapore and Malaysia), "BBC Select" (for North America) do not carry repeats of WN programmes like BBC iPlayer. I would gladly subscribe to a provider if they do carry WN programmes~)

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From the logistical aspect:

As some people mentioned above, it would be logistically difficult for WN to take Newsnight without making modifications to the show's format. 

If we were to take Newsnight live, the UK viewers on BBC Two will see awkward opt out points like "We now say goodbye to viewers around the world" at 2256 every night. Do you really want to see a news-making interview being cut off in the middle for an awkward opt out point?
 
If we were to take Newsnight on a repeat, say at 2330, the staff would have to truncate and edit a WN version of the programme almost immediately after broadcast every night. Not only does it add extra cost, but I think it's very hard to truncate a programme every night to exactly :26 minutes... You have to know when to cut off interviews or packages without ruining much of the context...  

This logistical problem applies to ALL suggestions here who wants WN to simulcast/take domestic programmes from BBC One/Two.