Why was I not given a reason as to why my daughter's Apple ID was locked?

The most common reason for the Apple ID to be locked is that the Password/Passcode was entered incorrectly too many times. Ask your daughter if she made these attempts or possible another person had her iPhone and was attempting to access it.

Changing the Apple ID Password/Passcode should be all that is needed to be able to use the device again. If you are the Organizer of a Family Plan, then you can change her Password from your device.

Change your Apple ID password - Apple Support

Otherwise you will need to follow these steps to change the Password/Passcode.

If you forgot your Apple ID password - Apple Support

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

Nobody on this forum works for Apple and we do not know anything more beyond what Apple says in its documentation and what people claim when they post on this forum. Apple tends to be pretty secretive (for whatever reasons they may have) about locking accounts unless they state so in the notice you get.

As was stated earlier several times, "security" reasons is fairly common. Changing the password is the resolution. Otherwise general account closure is mentioned in: Legal - iCloud - Apple

VII. B.

"Apple may at any time, under certain circumstances and without prior notice, immediately terminate or suspend all or a portion of your Account and/or access to the Service. Cause for such termination shall include ... "

Again, nobody here can answer your question because we do not know and we have no more access to your account information than you have. We are just other users like you. Apple is the only one that knows and if they won't tell you then you are up against a dead-end, sorry.

As for the device itself, Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Check the requirements and steps at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes proof other than it “must include the product serial number, IMEI or MEID.” Apple may typically require the original receipt* issued by Apple or an Apple Authorized Reseller. If Apple resets the device this will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

*Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support

If one of the messages you see are listed in this Support Article that mentions "Security Reasons", it is due to the Password entered incorrectly too many times.

If your Apple ID has been locked or disabled – Apple Support (UK)

If your daughter is part of a Family Sharing plan where you are the Organizer, you can prevent attempted changes to the account on the phone by going to Settings > Screen Time > Content & Privacy Restrictions > Account Changes. This will prevent your daughter from being able to sign out of the Apple ID on the device and also prevents another person from being able to log into her phone with their Apple ID that will then lock the device to that person.

Without providing the exact message you received, the only option we have is to guess what that could be. As you have seen, there are many different Locked messages where the reason and solution will be different. This also includes the Activation Lock where another person is allowed to enter their Apple ID to lock the device. Since you have already contacted Apple, there is nothing more that any user can provide as we have no access to your account to determine any reason. We also have no way of knowing why Apple is unable to tell you the reason or what action was performed on the device immediately before it was locked. That is the one piece of information that would be needed to prevent it from happening again.