Why some blocks didn't use full potential of their size? Currently there is 73600 Unconfirmed Transactions. But some blocks has only 749 kB or 0,21 kB or two block in row has 33 and 173 kB.

Why this blocks does not have 99x.xx kB? I mean there is a space for other transaction, why it is not include to this blocks?

Blocks:

440216 - 33 kB

440217 - 173 kB
Probably because a winning nonce/hash was found before they were able to fill up with transactions. There's no incentive in Bitcoin to make blocks fill up with transactions. In fact, there are blocks with no transactions (besides the coinbase one)!