MPMC

There's a lot of truth in that.

7 and 9 are odd numbers.

what is the problem with 7 bit it will just have a range of 2 to power 7 ?and will use 7 flipflops and 7 data buses.

Cheese... read my reply s_l_o_w_l_y
There is no problem with any size of data bus other than that you can olny use what is available...
As I say to the missus when she complains that I didn't buy some specific item at the shops.

Ya cannae buy what they dinnae have...
Del

No, it has to do with power generation.

Power is generated in multiples of 11Kv and that is not evenly divisable by 7 or 9, so it would imbalance the system and make it fall down and go boom.

The reasons are purely historical and ones of convenience.

4 bits 8 bits rapidly became the norm and so it has remained.
Of course you con't need to use them all. I manage to write small pump control systems generally using only 8bit integer arithmetic with timings in seconds which gives a maximum pump run time of over 4 minutes which is generally fine. I ocasionally use 16 bit words which is plenty big enough for me.

Unfortunately modern electronics is getting more designer hostile. Of course this is all under the guise of 'easier'. Chips are so small you can't bread board them. High level languages are just more complex and inappropriate for small applications unless you are using standard peripherals. Technical support is non existent.
Dunno where the next generation of electronics designers will come from?
But hey I s'pose they see all this stuff as 'normal' 'cos they weren't here in the good ol' days of real transistors.
Del