Like other people here have said, most of the reason will be the wear it puts on machinery and tooling equipment when used, and the other reason is probably the fact that the majority of the market would prefer a steel that was stainless (myself included honestly), so there is a smaller market for it than most production companies would like for some of their designs.
I would like to see more blades in M4 as well, but I also don't like taking care of steels that corrode easily, and patina is not my thing, so I have to stick to blades with a coating on them when I want M4. I'm thinking about getting a Gayle Bradley and having someone cerakote the blade for me, because I really love the design, but the satin finish has been holding me back.
Another thing to take into consideration is the fact that M4 is not as widely used in general, and so it is not as easy to get a hold of large amounts of it in general. In addition to the higher production costs, the raw steel itself is more expensive and less available than other alternatives like S30V, S35VN, VG10, etc.
Not only that, but when Spyderco has a knife made in Taiwan with M4 steel, they have to ship a load of the steel over to Taiwan in order to do that, since Crucible CPM-M4 is made in the US. They also have to do this for Carpenter's steels, but they can ship those over in larger amounts since they have more shipments from Carpenter anyway. Spyderco doesn't use as many Crucible steels now besides mainly S30V, so they don't have as many opportunities to send M4 over to their production facilities out of the country I imagine. It would likely be a smaller reason in general for them, but it's something to think about as well.
