@taxsavvy , Namaste. Please forgive my delayed response.
In order to facilitate and track tax collection, IRS uses TIN ( Taxpayer Identification Number ). Thus you have SSN, ITIN, ATIN, EIN etc. )
To be eligible for ITIN , one must have a need for filing a return :
1. beneficiary / taxpayer for US sourced income while still a Non-Resident Alien on work visa ;
2. beneficiary / taxpayer of US sourced investment income/ business with no work visa or even visitor visa
3. partake in the filing of a return ( Non-Resident Alien Spouse for joint filing with )
4. a minor dependent of a work visa Non-Resident / Resident Alien
Your mother's case creates an unique situation, in that she has no need to file a US return till she becomes a US person ( citizen / GreenCard). She has no US income, she is not an investor and even though she can be dependent -- there is no tax benefit i.e. personal exemption is no longer available , nor is there any tax credits to be availed. Under the current circumstances as described in your note , absent her investing in a US business, there is no need for her to file as a taxpayer.
See the instructions for form W-7--> https://www.irs.gov/pub/irs-pdf/iw7.pdf
I think that is why she is not eligible for ITIN.
Does this answer your query? Is there more I can do for you ?
Namaste ji
pk
