When the singing voice is also the regular voice

In animated musicals (or animated films with some singing but not to the point of being a musical eg Wreck-It Ralph 2: Ralph Breaks The Internet), sometimes the singing voice of a character is going to be done by a different actor/actress from the regular voice.

I'm going to give some examples from Disney.

1990's Disney: In Mulan and Aladdin, Lea Salonga does the singing voices of, respectively, Mulan and Jasmine. However, she doesn't do the regular voice for either. In The Lion King, Simba and Nala have different regular voices from singing voices.

2010's Disney: In Frozen, most of the singing voices are regular voices and vice-versa. Similar for Tangled.

Question: What are factors contributing to the decision whether to use 2 voices (1 regular, 1 singing) or just 1 voice (for both regular and singing)?

Edit: To clarify, i really have no clue. i'm not from the acting world. i don't have experience auditioning for acting jobs or anything. i have no idea actually which one is more seamful/less seamful/more seamless/less seamless or why. i just notice aladdin and mulan have different reg & singing voice while for frozen it's the same. no idea why this is the case and so i'm asking why are some are different and some are the same. i really have no clue about the economics, ethics or filmmaking decisions involved in this. I just observe something. I notice there's a difference. I'm just asking the cause for the difference.

2 competing models based on comments: it seems like they prioritise regular voice over singing voice. makes sense. but which of these 2 models exactly?

1 - if we could somehow quantify regular voice acting by ratings like with elo or glicko or something, then my thought goes like this: one guess is like if their regular voice acting rating is like 2600 but their singing rating is like 1500, one might ask like why not get someone instead, if possible, like voice acting rating of 2550 but singing rating is 2300. like my thought is that (2550,2300) is preferable to (2600,1500), but if there's none really available after a lot time of searching already, then let's get another person to do the singing. or if the next best is (2300,2300), then this is 200 points down already for voice acting rating, so perhaps let's get a separate actor for singing voice.

• quantitatively: it's like we have a utility function of, example, U(x,y)=9999x+y, with x as acting and y as singing. In this case, (2550,2300) gives more utility than (2600,1500). But of course it's easy to come up with utility functions where (2600,1500) is preferable to (2550,2300).

2 - based on comments, it appears that it's something like 'singing-blind' in re the 'need-blind' in university admissions: Basically, for live or animated (note that my question is for animated), the filmmakers or whatchamacallit don't care about the singing voice at 1st, then if the actor happens to sing then great and then otherwise they find someone else.

• quantitatively: not sure how to explain this. something like 1st maximise V(x), utility from acting and then 2nd maximise W(y), utility from singing. Or perhaps 'W(y)' should be like 'W(y|x)', maximise utility from singing conditioned on acting. or something.

Note 1: Of course it's impossible to quantify acting or singing ability. What I'm trying to ask specifically now based on comments below is if people may consider going with their 2nd best choice of acting for better singing or if people will necessarily go with the best choice for acting regardless of singing.

Note 2: There's utility U(x,y), V(x), W(y) and then there's money. What I notice is missing from answers and comments is the money involved. So the models might need to be changed from just maximising utility (which I guess is equivalent to maximising revenue) to maximising profit: so like P(x,y)=U(x,y)-C(x,y). How much more is the cost to get 2 actors instead of 1? Is it going to be worth the increase in utility/revenue? I don't believe these were addressed in comments or answers.