Because we don't have a big team of developers like Mojang do. And unlike Mojang we can't afford to spend all our time on this.
It will be good because PMMP will be as good as realms. All blocks and items are there, all biomes in worlds, sounds, redstone and minecarts, animals mobs, etc.
1. Why is it so hard for PMMP to add all the rest of blocks and items? 2. Redstone and minecarts are not working.
Its really easy for them to add all the blocks and items, but its harder to add the functionality. Also blocks and items are undergoing a (slow) rewrite currently. Furthermore minecarts are entities, and THE ENTITY SYSTEM IS COMPLETE B*******. It will be rewritten at some point.
No, it means it will be finished at some point in time. That could be anywhere between tommorow or 5 years from now.