Why does bitcoin mining take so long? I was wondering that a block (Here's a Block #506398)

It's nonce is 1,229,291,212 (A little over a billion), Then why does it take so long to mine a block because my system alone can do about 500 GH/s (500,000,000,000 Hashs per second), Which is very little in comparison to what an ASICs can do.

Then why does it take so long to mine a Block??

Thanks for your help!!
The entire purpose of proof-of-work mining is to force participants to spin their wheels doing busywork.  It takes about 10 minutes to mine a block because the difficulty automatically changes so that it always takes about 10 minutes to mine a block.  When more hashing power is added to the network, the difficulty increases to accommodate.  The time it takes to mine a block is intentionally set to 10 minutes.

You should read What exactly is Mining? for clarification.