I have read a lot of computer architecture textbooks, and I wonder why most of them (if not all) used MIPS as the architecture to teach. Why MIPS and not Intel or AMD or something else? What makes the architecture suitable for teaching?
The MIPS architecture is derived from an architecture specifically designed at Stanford for educational use and for research into CPU ISAs and architectural implementations. Early academic RISC architecture were designed such that they could be implemented (included layout) by small teams of graduate or upper-division students (not the cast of thousands involved in current Intel CPUs.) Thus MIPS may have basic features more compatible with textbook pedagogical explanations, and less legacy-derived "stuff" that isn't, compared with other commercial architectures.
MIPS may also have more available open source implementations very similar to it than other RISC or currently popular architectures.
Intel and AMD have a large share of the Desktop market. Other processor families are much more common in devices outside that fairly small area of influence. MIPS is also a RISC architecture which are generally thought of as being easier to learn. You learn a small number of commands that can be combined orthogonally a multitude of ways similar to UNIX style piping.
Computer Architecture books that are educational rather than specificational are generally geared toward concepts rather than specific implementation details and thus tend to be more likely to use RISC over CISC simply because there's less to teach in order to understand the instruction set.
I do expect a convergence with ARM though as it is an extremely widely used architecture that is also RISC so I'd think it will show up in architecture books soon enough.