I have question upon mikroC (it's kind of different with C), and I cannot create a new tag, so I was wondering why users with reputation points less than 1500 cannot, at least, suggest a new tag to be added to his question.
Probably because this is not a very common scenario. New features are implemented when the higher-priority features are done, so if you want this new feature I suggest you start a new question (or edit this one) tagged with feature-request, where you explain what you want, and maybe give a rough workflow of how this could work.
It took a while before they made it possible for everyone to (suggest an) edit, because this is hard to get right (find the right mix between easy editing and preventing abuse). From a global view it does not seem there is a shortage of tags, and maybe this could do more harm than good ("tag pollution" is a hard thing to keep under control). There is also a simple "workaround": end your question with "Please add the mikro-c tag to this question, I could not create it myself", and a higher-rep user can edit your question and add the tag for you.
You could simply add a comment to your question requesting that tag. If a higher rep user thinks it's reasonable he'll edit it into your question.
If you really think it is a good tag (and otherwise you shouldn't create it anyway), then you can consider writing a post on the per-site meta with your rationale why it is a good tag and with some examples where it could be used.