Assembly language is defined by the assembler, the software that parses the assembly language. The only "standard" is the machine code, that has to match the processor, but if you take 100 programmers and give them the machine code standard (without any assembly language hints) you will end up with somewhere between 1 and 100 different assembly languages. Which will all work perfectly well for all use cases of that processor (baremetal, operating system, application work) so long as they make a complete tool that fits in with a toolchain.
It is in the best interest of the the creator of the instruction set, the machine code, to create both a document describing the instruction set and an assembler, the first tool you need. They can contract it out or make it in house, either way doesnt matter, but having an assembler, with a syntax, and a document for the machine code, which uses the assembler's syntax to connect the dots between the two, will give anyone possibly interested in that processor a starting point. As was the case with intel and the 8086/88. But that doesnt mean that masm and tasm were completely compatible with intels assembler. Even if the syntax per instruction matched, the per instruction syntax is only part of the assembly language there is a lot of non-instruction type syntax, directives, macro language, etc. And that was from the DOS end of the world, there was the UNIX end and thus AT&T. gnu folks at the time were unix end of the world so it makes perfect sense that they used the AT&T syntax or a derivative of as they generally mess up assembly language during a port. Perhaps there is an exception.
nasm and some others like it are an attempt to continue the masm syntax as masm is a closed sourced Microsoft tool (as was tasm and whatever was with Borland C if that wasnt tasm as well). These might be open sourced now but no need, easier to write one from scratch than to try to port that code, I assume to be built with a modern compiler, and nasm already exists.
The why question is like asking my why you chose the pair of socks you chose this morning or any particular day. Your socks may not have as big of an impact on the rest of the world, but the question is equally irrelevant and/or unanswerable. The answer goes back in part to the ask 100 programmers to make an assembler for the same machine code definition. some of these programmers may be experienced with assembly language and may choose to create an assembly language in the image of one they have used before which means several of them will make one that looks pretty similar to each other. But the one or ones they used before may be different so there would be groups of these similar but still different. Then in lets say 30 years ask each one of those 100 people the why question...if they are still alive...Like asking me why you chose to declare a variable in a program you wrote 30 years ago in the way you did it.
