Why do transactions take so long to confirm in testnet? I have created a transaction with high miner's fee and it is now in mempool (in testnet):

https://live.blockcypher.com/btc-testnet/tx/abcc4ce473f490896e87102fe3ffb857ff735a0fc97525dadf9c5b0ed5410f9f/

However, it doesn't seem to be confirming. It's already missed two opportunities to confirm. Transactions with much lower miner's fees are going through instead.

Is Bitcoin this random or am I missing something?
Decoding your transaction shows an output:

"vout": [
    {
      "value": 0.00000000,
      "n": 0,
      "scriptPubKey": {
        "asm": "OP_RETURN 68656c6c6f5f776f726c64",
        "hex": "6a0b68656c6c6f5f776f726c64",
        "type": "nulldata"
      }
    }, ...


This might be the reason the miners won't touch it as it may be flagged as non-standard on older nodes. Notice the warning on blockcypher:



Maybe someone can comment on exactly why, as these null_data transactions were made standard on Bitcoin-core 0.9.0, see https://bitcoin.org/en/developer-guide#null-data