Quicken for Mac why don't registers have 2 lines?

Well, that's a pretty loaded question, with a lot of viewpoints over the years.

When the predecessor of the current Quicken Mac was developed back a decade ago, the developers decided to use some of the built-in tools of the Mac operating system to power the register views, rather than doing extensive work to hand-code the display of transaction data as had been done in the legacy Quicken Mac 2007 as well as Quicken Windows. They didn't think they needed to stick with the checkbook register design, as increasingly people were not using checkbook registers, and by using the operating system to do most of the work to power the display of rows of transactions, they were able to build it faster, make it work razor-fast and scroll super-smooth, and build in additional functionality to the registers. 

I'm a longtime Quicken Mac user, and I'd say that I and most Quicken Mac users from the past found the single-line display jarring and distressing at the start. Over time, some people still wish for the 2-line display while others -- like me -- have gotten used to it, and now even prefer it to the old 2-line display. (I occasionally launch my old Quicken 2007 and I now find the 2-line display in that program jarring now!)

The single-line display in Quicken Mac makes it easy to show or hide whatever columns you want, put them in the order you want, and adjust the widths as you want. Personally, I find it much easier to skim up and down a screen looking for something without every other row containing differently-aligned different data.

So the answer to your question is a combination of technology, resources, and personal preferences. The developers have said it would be a complex and time-consuming project to program a two-line register display, and they apparently have felt that there are other missing features in Quicken Mac which were higher priorities to work on. There are a fixed number of programmers, and their time must be carefully allocated to the enhancements the developers feel are most meaningful/helpful to the largest number of users.

And while many new users initially have a knee jerk reaction asking for the 2-line display, I think they also really wanted new users to Quicken Mac who were used to the 2-line display in legacy Quicken Mac or Quicken Windows to try using the single-line display for awhile to see if they continued to find it problematic.

That said, there is an Idea post on this forum where you can add your vote for a 2-line display if you feel it's something which should be a priority for development. It is listed as "Under Consideration", which means the developers have not decided whether or not to put this on the development roadmap.