I wonder what was the real reason behind this. Surely taking the iOS's Weather app and scaling it to bigger screens didn't take them 12 years. Anyway, I'm glad it's there.
Federighi said it would be very easy for Apple to just scale the iPhone Weather app to the iPad, but that's not something the company wants to do, instead looking to create an app that makes use of the larger screen of the iPad in every possible way
[1] https://tech.hindustantimes.com/tech/news/apple-exec-explain...
Either they explicitly didn't want to make one for some reason or they just didn't care. If they actually wanted to make a weather app they could have done it.
There have always been far more important things to work on than an iPad weather app. In this case they happened to get it for free with their Dark Sky acquisition.
For example, the Xsnow accumulation on the top in-app widgets! I wonder - if you shake the iPad, will it clear away?
Why it didn't come with a weather app at release in 2010 I'm not sure, but why it hasn't come in the past few years is almost certainly because they were building it with their new in-house weather API.