It is simple, Rogers and Bell hold rights in Canada for different content. Netflix does not have the rights to sell that content in Canada.
But the content is easily available to Canadians by bypassing geoblocking therefore this makes very likely than Netflix is gaining extra Canadian customers and Canadian providers losing them. Technology exists that would enable Netflix to stop this practice but it chooses not to do it. Netflix could be breaking copyright laws by doing that and if this would end up in court, anything could happen.
But Bell and Rogers don't own the content, just the distribution rights. Here's an example of how this works:
Bell owns the exclusive distribution rights to HBO content in Canada. However, if HBO were to sign a second agreement allowing Rogers to air Game of Thrones on FX, in violation of that agreement, Bell can sue HBO, but they can't sue Rogers. That agreement is between Bell and HBO - Rogers isn't involved. Bell could attempt to get an injunction to prevent Rogers from airing the show, but ultimately it's HBO who would be responsible for making Bell whole. (Rogers could also sue HBO in this scenario, but that's not really relevant).
So let's extrapolate this out to Netflix. Let's say Bell has exclusive Canadian distribution rights to a Showtime show, but Showtime has also signed a distribution deal with Netflix UK. Now, if by some miracle, Bell was able to establish in Canadian court that Netflix makes it so easy to access other regions that making those catalogues available to Canadians is an intended component of their service (which, by the way, I don't think they can establish), then they still can't go after Netflix. The only one they can sue is Showtime for violating their exclusivity arrangement by licensing their content to a different provider who offers service in Canada. They could also attempt to get an injunction against Netflix UK, but I'm sure you can see why that is completely unenforceable.
So their only other avenue is to attempt to make circumventing geoblocking illegal. And if we put aside for a moment the fact that this won't happen and is no more enforceable than torrenting movies being illegal, they still can't use that to go after Netflix. The only ones violating the law would be the users of the circumvention tools, not Netflix. In fact, that would further insulate Netflix, since they can argue that it's law enforcement's job to catch those people, not theirs.
There is only one way Bell and Rogers could get Netflix directly. If they were to license some content they own for Netflix to distribute only outside of Canada, and they were able to establish in Canadian court that Netflix makes it so easy to access other regions that making those catalogues available to Canadians is an intended component of their service (which, by the way, I don't think they can establish), then they could sue Netflix for copyright violation. In which case, Netflix would pull the content, and that would be the end of it.