You can query data from SQL; just include necessary NPM driver and you're away. It wont be "Realtime" though.
Other NoSQL Support: The realtime driver is the problem here.
Meteor creates "live queries" by tailing the MongoDB oplog. This works in part because "Minimongo" a javascript based mongo query engine can examine the oplog and determine what does & doesn't match a live query.
To support another NoSQL solution, this huge engineering effort would need to be reproduced.
To support "Realtime" SQL, they'd need to hook in to something like PostgreSQL 9.4's logical replication, and have a javascript based query-compiler & interpreter. (Or provide a mongo-like interface to SQL.) Not impossible, but not easy by all means.
I somewhat agree with the Grandparent (re: Firebase/Parse) - However, I'd like to see the Meteor team have their own realtime database; as the realtime paradigm changes how a lot of it would be built.
The lack of out of the box other NoSQL options is puzzling though because NoSQL db's are a lot close to each other than pure SQL is. And given the general disdain for MongoDB these days, it seems that uncoupling from that dependency would be a high priority. I'm sure there are plenty people on HN that simply won't even give it the time of day because of MongoDB.