Alright, I'll give it a punt.
Why are the Left not concerned about...
"mass immigration and the detrimental effect on our infrastructure"
Because I think taken as a whole, the pros of mass immigration outweigh the cons. This isn't to say some aspects of our immigration policy are not wrong or badly implemented - both of those things are true. But mass immigration in principal is not something I see as a wholly negative thing.
"the invasion of Europe"
Because I don't think Europe is being invaded
"the threat of Islam on our culture"
I dislike Islam very much, I think the religion is in dire need of a reformation, and I think there are genuine concerns about integration, some of which have genuine security implications. But I don't see Islam itself as an existential threat per se. I don't think it is going to wipe out British culture, even if there are a handful of Muslims who want to do just that.
"the increase in knife crime"
I am concerned about that - I'm pretty sure most people who would consider themselves as being on the left care about crime, too.
"the changing face of Britain? "
Because I don't see the changing face of Britain to be innately anything to be concerned about. On it's own, it doesn't bother me.
Cue accusations of delusion etc. - doesn't bother me. If you want to know what this particular individual - who if asked would call themselves left wing - says in response to your questions, these are your answers.
People on the left, when you get down to basics, want most of the same things that people on the right want. They want a society that's fair, peaceful, and just, and they want a foreign policy that protects UK interests in a way that has moral integrity. People on the right want exactly the same things. Difference is, each group of people is working on very different definitions of what those words mean and very different beliefs about how they can be achieved.
It's also worth mentioning that "The Left" is not a real group of people. It's a very very very very loose umbrella term that describes various different ideologies, some of which have very very little in common with each other at all. Exactly the same is true for "The Right". So throwing these terms around in this way makes you sound a bit like a teenager who has learned about politics through youtube and now thinks he is an expert.