Why doesn't Minecraft Pocket Edition and Minecraft Pc edition not have the same content?

The devices that run pocket edition simply don't have the processing power of a pc.
Also, many more people play on the pc, so they tend to roll out updates for the pc first to meet the majority of demand.

The disparity between the PC version of Minecraft (hereby known as "Minecraft: Java Edition" or "Minecraft Original") was created in Java & doesn't have the same constraints, so content can be added a bit easier.

Minecraft: Pocket Edition (hereby known as just "Minecraft", as it'll be merging with the XBox One, Nintendo Switch & WIndows 10 Versions with the "Better Together" update by the end of 2017) was a remake of the game from the ground up, swapping out Java for C++ as it's coding base & running on a different engine (which will be the Bedrock Engine with the upcoming "Better Together" update). While the change is the coding base & engine better optimized the game for the other platforms (which are more tightly constrained), it causes a developmental split where new stuff comes out on the Java version first (which is primarily done by Mojang) & other versions later (Pocket Editions by a second team at Mojang & Console Editions by 4J Studios). Since the non-Java versions require extra coding, QA testing & certification before updates are pushed on their respective platforms, there will always be an arms race between Minecraft & Minecraft: Java Edition.

On a side note, I must mention that the Playstation 4 version of Minecraft is NOT noted in the "Better Together" update due to Sony's reluctance to permit online cross-platform play with users outside the Playstation Network (so far, the only current game that has SOME cross-platform support is Rocket League… Which is ONLY PS4 to PC ONLY. XBox One, Switch & PC versions have full cross-platform support). If Sony ever decides to change their mind, they'll get added in later.

Hope this sheds light on the subject.

Because they want money