Why Bitcoin-qt cannot read my wallet whereas blockchain can

[lepko]

I have first installed bitcoin-qt on a windows 8 tablet with very small space available. So my disk was filled up soon and bitcoin-qt jerks.

I transfered wallet.dat to another computer but bitcoin-qt  wasn't able to read my wallet by flashing an error.

Then I read on a forum someone used blockchain and it works also for me.

Why can't bitcoin-qt repair my wallet whereas blockchain can ?

Still if I would like to use my wallet on my pc so how can I get my wallet back at home from blockchain ?