You are one of many (many!) that are hoping for Instagram for WP8. It's a great app, a lot of fun, and essential to a lot of people to stay connected to friends. It's arguably the most glaring missing ingredient in the WP8 app selection. I don't think anyone can really answer the question of why it's taking so long to develop the app for WP8. However, I have to imagine it's only a matter of time.
There's no doubt that Microsoft and Nokia are quite aware of the desire for WP8 Instagram. In fact, as pointed out above, Nokia launched an app called #2InstaWithLove that allows people to snap photos, apply a vintage-filter, and then post to social media sites w/ the hashtag automatically attached. (Do a Twitter hashtag search, for e.g., you'll see some cool photos.) This is supposed to illustrate to the "players" involved how much passion for Instagram there is among the Windows Phone customer base.
All I can say is, if you need a new phone at this very moment, and Instagram is a deal-breaker, then I'm afraid you're out of luck. Furthermore the rumor mill is very quiet. I've heard one mention of May as a possibility, but there's no substantiation from anyone.
However, if you can hold out a little longer, I think you are definitely on the right track with WP8, and you will find it's worth the wait. It's a really enjoyable OS with really nice handsets, too. Spend a little time on the Microsoft Windows Phone site and check out the apps. There are some great apps that no one really talks about. Awesome games, productivity tools, art apps, etc.
As a final side-note, if you enjoy photography, WP8 is a wonderful OS. Many of the phones have terrific cameras, and there are some fantastic photos apps available. Filters, editing, panoramas, etc.