Quite possibly ...
... it is not about neglecting the number of people using Urdu or other languages with a large user base. Maybe MSFT made an assessment of how widely used iPhones (and Outlook on iPhones) are in the lands where those languages are spoken and weighed the potential losses stemming from pissing off some section of the market against savings on Product[*]/Development/QA work.
It would be interesting to know the corresponding statistics if anyone can dig them up, and also whether the same languages remain available for Outlook on Android.
[*] I suspect the Development and QA parts are intuitively clear to the commentariat, but in my experience people don't tend to give much thought to, say, the real estate on menus/labels/tooltips/etc. and what exactly will be usable/intuitive in every supported language until one actually encounters the problem in a multilingual setting. "Why is this input field so far right?" - "Because the French/Russian/Welsh/whatever label next to it is longer than the English one that you see right now" kind of thing.