Why are these two architectures incompatible?
The PowerPC and Intel architectures vary individually in their various iterations, and are in many ways incompatible. This means the instruction sets for each will not be compatible, as well.
There are a lot of reasons why any two given architectures will not be compatible (register number/size, memory allocation schemes, processing unit differences, bus width, etc.), but at a basic level the instruction sets for the two architectures are different. You can look at the differences between them; using an iteration of each at random, the PowerPC instruction set and the Intel instruction set have different opcodes. So, even within different generations of an individual architecture family there is likely to be some incompatibility between any given assembly program, as the underlying opcodes can be different.
So, this question is a bit like asking why isn't someone who knows a human language x able to understand an animal call y? The underlying languages and structures of both systems are essentially different, while sharing some functionality at a high level. There are examples where the two share functionality and can overlap or communicate (honey birds in Africa and people who are able to call to them, people who can practice calls for various species, human use of artificial calls, etc.), but generally they are only vaguely similar in that they are both "languages," and one would not expect them to be 100% compatible.