I was reading a question on this site, which I thought should be edited for better grammar and spelling. But I could not due to this limitation. Why such a limitation?
This is by design. Compared to their respective main site, meta sites are generally low-traffic, and there's less of a need to have all posts conform to a certain standard of grammar, formatting and overall presentation. Any glaring problems can be pointed out in comments or addressed by moderators or high-rep users, so adding suggested edits would do little except increase the workload of those who have to review them.
Without knowing the post you were referring to, it's difficult to advise, but per the privileges section of the Help Centre, you should be able to edit community wiki posts once you have 100 reputation.
You simply can not edit other post than tag wikis here at the Windows Phone Meta site. This seems to apply to all Meta sites at Stack Exchange.
Please see the picture below for reference. (pointer a la paint)