The name Islamic State is a claim to a global caliphate .
This claim is rejected by everybody else, from western leaders to Muslim leaders , to other Islamic extremists .
"so-called" is added to make it clear that this is not an objective or descriptive term, but a self-chosen label which does not reflect the true nature of a group.
It happens with other groups and organizations as well, such as "so called alt-right" to make it clear that it is a white supremacist propaganda term, or "so called GDR" to make it clear that it is not actually a democratic republic.
politics.stackexchange.com/users/8062/tim
Because they (Daesh) claim that their government is "Islamic State", but they consider most of Muslims as polytheist or infidel, and on the other hand, the majority of Muslims all over the world consider them to be fake Muslims.
We face with two facts:
On the one hand they (Daesh) are known in the media as "Islamic State", so in order to talk about them one will inevitably call them "Islamic State".
On the other hand, they are not really an "Islamic State" (not Islamic nor a state), so one cannot call them "Islamic State".
As a result, one calls them the so-called "Islamic State" .
Update: this part is explanation of "they are not really Islamic State".
In fact this is my deleted comment that answers a deleted comment that call Daesh as "strict Islam": No.
They are not "strict Islam".
It's cartoon of Islam, not strict Islam.
They read only war verses of Quran, not verses like: Say, "O People of the Scripture, come to a word that is equitable between us and you - that we will not worship except Allah and not associate anything with Him and not take one another as lords instead of Allah.
But if they turn away, then say "Bear witness that we are Muslims.
Qur'an 3:64 ...nor verses like...
There shall be no compulsion in acceptance of the religion.
The right course has become clear from the wrong.
Qur'an 2:256
They also don't consider many other verses that forbid killing people.
The Qur'an as a book should be taken as a whole.
politics.stackexchange.com/users/7739/user 1
This is a specifically BBC policy, that some other outlets are following.
The general policy at the BBC is to use the name for a group that is generally understood, and that the group itself uses.
When Daesh were first expanding in Syria and Iraq, there were t
wo names in somewhat widespread use : ISIS and ISIL, both of which are abbreviations of possible translations of Al-Dawla Al-Islamiya fi al-Iraq wa al-Sham .
The BBC chose ISIS.
Political leaders in the UK and US chose ISIL.
After Daesh had captured Mosul and declared independence the BBC started using "Islamic State".
The organisation was clearly in control of territory.
This seemed to be the term in widespread use.
In a radio interview, following the attack on the Tunisian beach resort of Sousse, David Cameron said: I wish the BBC would stop calling it Islamic State because it’s not an Islamic state.
What it is, is an appalling, barbarous regime …
It’s a perversion of the religion of Islam and many Muslims listening to this programme will recoil every time they hear the words Islamic State … 'So-called' or Isil is better.
Source
There were other complaints to the BBC, but it seems that the BBC did not want drop the term "Islamic State" completely.
So it was decided (presumably by someone quite high-up, perhaps James Harding, head of BBC News)
that presenters would prefix "so-called" before "Islamic State" for at least the first time each journalist uses the expression in a report.
Other news outlets have followed suit, the BBC has a lot of influence on British Media.
politics.stackexchange.com/users/8554/James K
French middle east specialist 'Jean Pierre Filiu' explains that it is one of the goal of their propaganda towards western countries to appear as a state, and the state where any good muslim should go.
(explained in French in this radio program :
He notes that this is the first time that a terror group name is translated into western languages (IS for english, Etat Islamique in french, ...), while other groups name like 'Al-Qaeda' (litterally 'The Basis'), were never translated.
So, western media have followed the policy in 2 rounds:
let's translate the name
: we understand what it means, so let's go, and Probably this was a little bit stupid, falling directly in their propaganda, let's add the 'so-called' before...
No media made the choice of going back to the arabic word which has no particuliar other meaning than designing a terror group.
politics.stackexchange.com/users/16360/Wilfried Maineult
The journalists are using term 'so-called state' to prevent controversies about the word 'state' which is used to describe a geopolitical unit.
ISIS as a government is not acknowledged by any western state (they are not even called 'partisans' or 'rebels', instead they are referred as 'terrorists').
Using the term 'Islamic State' without 'so-called' could be interpreted, as if a specific journalist would consider they as a government that at least could be a candidate for acknowledgement.
Saying 'so-called' makes it sure that you distance yourself from considering them a legitimate government, or sympathize with their claims.
In international politics, you need to be very careful about the names.
politics.stackexchange.com/users/57/Danubian Sailor
Often in news articles, I see that IS are referred to as "so-called" Islamic State. For example, in a  BBC News article :   On Bastille Day last year, along the coast in Nice, more than 80 people were killed when a lorry was driven into celebrating crowds on the seafront in an attack claimed by  so-called  Islamic State.   Why is this prefix used?
Why is Islamic State (IS) referred to as "so-called"?
politics.stackexchange.com
<terrorism><islamic-state>
politics.stackexchange.com/questions/23876
politics.stackexchange.com/users/16328/Cthulhu
Daesh claim that their government is "Islamic State", but they consider most Muslims as polytheist or infidel. On the other hand, the majority of Muslims all over the world consider them to be fake Muslims.   The Muslims there are considered to be polytheist or infidel and this is the reason "so-called" is used. 
The Muslims there are considered to be polytheist or infidel and this is the reason "so-called" is used. 