Because tables are implementation-specific extensions.
I find the dokuwiki table syntax to be pretty simple and effective, or the markdown extra version. Don't know why none has been made a standard part of markdown yet. Seems like it must be due to backlash against table-based web designs of the 1990s, (which resulted in an entire generation of web developers thinking that tables are inherently evil and an entire generation of non-developers who just do things in MS Word or Excel instead because it's relatively easy to make tables).
A standard can be descriptive (bottom-up) or top-down (prescriptive). A descriptive standard will only check and document common-ish features to implementations. Tables are not one of them.
[0]: http://web.archive.org/web/20140830092621/https://michelf.ca...
PHP Markdown, MultiMarkdown, etc also support that syntax. Stack Overflow is pretty much the exception [2].
I'm not interested in Markdown parsers which do not support tables. They are useless to me.
[1] http://www.reddit.com/about/ - "last month, reddit had 114,540,040 unique visitors"
[2] http://meta.stackexchange.com/questions/138946/can-we-add-ma...