Altium has taken over a lot of small to medium sized shops. Mostly because the price is right for its capability. It also has a history of being the least bad compromise between the odd mixtures of excellence and user-hostility Cadence and Mentor tend to come up with, going back to the Protel days, and they've done a good job in the last decade+ of marketing it to those shops. Cadence and Siemens nee Mentor (and maybe Zuken? I've never seen Zuken in the wild, but it always makes these lists) have been neglecting the entry level and smaller organizations and aggressively trying to move their customers to their higher tier offerings during that time. But while it's Altium's flagship product, it is not top tier. It is really entry-level for a professional PCB-level design package, like PADS and OrCAD as opposed to Xpedition and Allegro.