Questions asking for software recommendations are on-topic both here and on the SE site specifically for them. As Ubuntu is both on U&L and here. In fact it can even be more useful to have Ubuntu-specific software recommendations here as people will know where to find them, there is no need to migrate them to the site specialising in all software recommendations.
As it is not the case for Ubuntu questions on U&L.

• primarily opinion based (is GIMP a good alternative to Photoshop or not? And Krita? Wait, what about Cinepaint?)

 
• too broad as there can be many possible answers (What is a good text editor for Ubuntu? Go figure which one will get recommended...)

Software recommendation questions will be closed with one of these reasons if they are not specific enough to what exact features are needed. Or at least they should be in my opinion

An answer to the literal why question:

Because they are asking about software for Ubuntu and basically anything relating to Ubuntu especially the software that runs on it is on-topic here. That was just a thing decided in the rules. You are allowed to give specifications for Ubuntu software and have it recommended to you. Normally if you are specific too much then it's not 'too broad' and not just 'primarily opinion-based' (you should note that questions on software recommendations are closed with these reasons if they are not specific enough).

Questions asking for software recommendations are on-topic both here and on the SE site specifically for them. As Ubuntu is both on U&L and here. In fact it can even be more useful to have Ubuntu-specific software recommendations here as people will know where to find them, there is no need to migrate them to the site specialising in all software recommendations.
As it is not the case for Ubuntu questions on U&L.

• primarily opinion based (is GIMP a good alternative to Photoshop or not? And Krita? Wait, what about Cinepaint?)

• too broad as there can be many possible answers (What is a good text editor for Ubuntu? Go figure which one will get recommended...)

Software recommendation questions will be closed with one of these reasons if they are not specific enough to what exact features are needed. Or at least they should be in my opinion

An answer to the literal why question:

Because they are asking about software for Ubuntu and basically anything relating to Ubuntu especially the software that runs on it is on-topic here. That was just a thing decided in the rules. You are allowed to give specifications for Ubuntu software and have it recommended to you. Normally if you are specific too much then it's not 'too broad' and not just 'primarily opinion-based' (you should note that questions on software recommendations are closed with these reasons if they are not specific enough).

Questions asking for software recommendations are on-topic both here and on the SE site specifically for them. As Ubuntu is both on U&L and here. In fact it can even be more useful to have Ubuntu-specific software recommendations here as people will know where to find them, there is no need to migrate them to the site specialising in all software recommendations.
As it is not the case for Ubuntu questions on U&L.

• primarily opinion based (is GIMP a good alternative to Photoshop or not? And Krita? Wait, what about Cinepaint?)

• too broad as there can be many possible answers (What is a good text editor for Ubuntu? Go figure which one will get recommended...)

Software recommendation questions will be closed with one of these reasons if they are not specific enough to what exact features are needed. Or at least they should be in my opinion

Questions asking for software recommendations are on-topic both here and on the SE site specifically for them. As Ubuntu is both on U&L and here. In fact it can even be more useful to have Ubuntu-specific software recommendations here as people will know where to find them, there is no need to migrate them to the site specialising in all software recommendations.
As it is not the case for Ubuntu questions on U&L.

• primarily opinion based (is GIMP a good alternative to Photoshop or not? And Krita? Wait, what about Cinepaint?)

• too broad as there can be many possible answers (What is a good text editor for Ubuntu? Go figure which one will get recommended...)

Software recommendation questions will be closed with one of these reasons if they are not specific enough to what exact features are needed. Or at least they should be in my opinion

An answer to the literal why question:

Because they are asking about software for Ubuntu and basically anything relating to Ubuntu especially the software that runs on it is on-topic here. That was just a thing decided in the rules. You are allowed to give specifications for Ubuntu software and have it recommended to you. Normally if you are specific too much then it's not 'too broad' and not just 'primarily opinion-based' (you should note that questions on software recommendations are closed with these reasons if they are not specific enough).