By definition, the English left wants to reform the British state.
Short of the breakup of England into independent statelets (not a likely or widely desired prospect) or a violent revolution (likewise), they are stuck within existing structures. Any change must come from incremental reform of those structures. Historically speaking, Scotland and Wales have made a strong contribution to progressive reform of the UK. English leftists hope this will continue in the future.
By definition, Scottish nationalists want to leave the British state.
The inclusive, civic, mildly socialist strain of Scottish nationalism is admirable in its way; but it deliberately contrasts itself with the rotten condition of Westminster. These nationalists believe attempting to reform the UK is not worth the effort. Instead, they want to set up a shiny new state of their own.
Maybe their analysis is correct about what is best for Scotland. There are certainly rational arguments to support it. However it carries an implicit message to English reformers: We are not committed to helping you. Our priority is to get out. We wish you well, but in the end, any aspirations you may have for England are not our concern.
When you put it like that, it is hardly surprising the English left regards Scottish nationalism with distrust and hostility.
If you ask the English left to support Scottish independence, you are asking them to sacrifice tangible interests for a point of principle -- specifically, the natural justice of an independent Scotland -- with which most of them do not agree in the first place. No matter how you spin that, it's a tall order.