Another reason you see a lot of walleye and salmon fisherman going with lowrance is there ability to interact with more tools that those fisherman need. Radar is a prime example. Although I am not familiar with the Hummingbird enough to comment on their ability to connect with Radar, I can tell you that lowrance makes it's own Radar Array that connects to their devices. As a salmon charter or big time walleye angler or tournament fisherman, Radar is a must to ensure safety of yourself and your crew.
Hummingbird connects with Minn Kota is another reason you might see more Birds. Lowrance with Motorguide. What we have found however is that the Motorguide doesn't do everything we want it to do in the walleye community. Many members that switch over from Minn Kota just to connect their device often switch back within a year just because of what they are missing. The key issue is the Motorguide FOB doesn't have a display screen. Although not a big deal if you are operating from your Lowrance device, it IS a big deal when you make adjustments direct from the FOB. You cannot tell your motor settings!
Now, a lot of guys are running duel screens at the helm. Some are using one specific technology (like Lowrance) and running mapping on one while running down imaging or 2d sonar on the other. They might run and HDS7 and an HDS9. I prefer the bigger screens and run split or quad screens. I also have fish finders in the back of my boat as I am trolling and the majority of the time, I am looking at the back and do not want to look forward at the helm. We are now starting to see guys purchase a Hummingbird specifically to operate their TM while using lowrance for other features like mapping.
Crazy world!