Because he’s an intermediary to provide intel and wiretapping thru his connections across the pond. If you can’t obtain a FISA warrant (which they didn’t several times), you have to create the evidence to get one (Don Jr. meeting & dossier). While doing that (takes time), a foreign entity can circumvent the US legal process by conducting surveillance and sharing this info thru partners such as Fusion. I believe this is why Robert Hannigan suddenly resigned from British GCHQ on 1/23/17 just a day or two after Trump was sworn in. I believe the part that hasn’t really come to light yet is how a foreign government assisted in this mess.
Could be. But we still don’t know. For instance, how do we know that Steele didn’t have a greater/different role than the one that’s being suggested/alleged? We just don’t bloody well know. And I’m concerned that if folks just believe what’s being spoon fed to them, they’ll only look for evidence that fits the prevailing theory and not the theory that fits the evidence.