I think "advice" and "suggestion" are both abstract nouns. Then why is "advice" used as an uncountable noun and "suggestion" used as a countable noun?
For the same reason "the pen" in Spanish is "la pluma", while "the pencil" is "el lápiz" instead of "la lápiz": it sounds more natural to native speakers (as StoneyB mentioned in the comment, we're 'voting' on how to use these words).
Note that if you mix these up you'll probably get a few funny looks, but you won't be misunderstood.
If you ask for advice, you really don't know how much advice you need, otherwise you'd probably wouldn't be asking for advice. Advice when desired is usually not a precise, measured activity, the corporate world's abuse of the term advise notwithstanding (and even then, no guarantee that it's precise/measured there either).
When you ask for a suggestion, it's focused and directed on a certain thing, so it's measurable to a greater extent. Give me two suggestions on where to go - the asker wants you to respond with two places. Give me advice on where to go - the asker really isn't specifying the number of places, because he or she doesn't really know. You could respond with You need to go here or You need to go to place A, then place B, and finally place C.