They dropped it because the world changed, I suspect. iPhoto was a key element of what Apple called the Digital Hub strategy - your Mac was the centre and other devices were fed from it. Now, the Cloud is rapidly becoming the centre. Now there are many people who have devices (Phones, Tablets) who rarely use or maybe don't even own a desktop. Also the volumes of photographs taken has increased exponentially.
So, one of the consequences of the volume of photos is an increasing demand for automation. Folks want to shoot and look at their pics, but don't want to spend time organising and editing. Result: filters for editing and automatic organisation like Moments. (BTW, Apple are not alone in this. The new photo service from Google is similar.) There are still manual options - albums, keywords and so on - but the front and centre features are those automatic ones.
The Cloud becomes the centre because people want to shoot with their phones, run a few filters and show them to their friends or share them. The cloud facilitates all of that, and again, automation is the key. Volume you see. Folks don't want to spend time organising...
As phone cameras have improved - and they have - sales of point and shoot cameras are falling and dSLRs are pretty static. The dominant photo device of today is the mobile phone, and Apple make rather a lot of them, and those are the folks that Photos.app is designed to serve.
But again, not exclusively. There are features in there that will work for the people who like to import/edit/organise and so on, most of the same features that were in iPhoto. But whereas automation in iPhoto gave you events, with limited flexibility, in Photos it gives you moments with none. So, use the manual tools - like albums, keywords, smart albums etc.
If, like me, you don't feel the app is for you, then use a different app. Personally I have an ongoing love-hate relationship with Adobe's Lightroom. I shoot with a dSLR, others in my family shoot with a P&S and phones. All are managed in LR. But Photos is free and LR isn't.
Which app to use depends on that features you require in your manager. For instance, if you want to use iCloud sharing then you're pretty much stuck with Photos. But there are other ways of sharing.
Ask yourself, what are the features of iPhoto that you miss. What ones would you like. Answer those questions and you'll be well on the way to finding a replacement.
