Why low cpu usage on sql server developer edition 2014
SQL Server Developer Edition can use all available cores unless you've specifically configured it otherwise. Whether or not it will use all cores depends on the workload. 

If you are running a query like SELECT * FROM dbo.YourTable, it will likely be a single-threaded plan and only use one CPU. You'll need to run multiple instances of such a query to see more CPUs used. 

A single query that uses parallelism should be able to use multiple cores. The example below uses all 12 cores of my box, although total CPU averages about 40%.

WITH 
     t10 AS (SELECT n FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) t(n))
    ,t1k AS (SELECT 0 AS n FROM t10 AS a CROSS JOIN t10 AS b CROSS JOIN t10 AS c)
    ,t1g AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) AS num FROM t1k AS a CROSS JOIN t1k AS b CROSS JOIN t1k AS c)
SELECT COUNT(*) AS CountOfRows
FROM t1g;