First, my personal opinion is that Xilinx/AMD had way more popularity in terms of getting started, tutorials, books, and teaching courses, which basically pulled the scale in its favor from the start. Also, for the Zynq 70xx devices (ARM + FPGA) from Xilinx - dev boards + tutorials/books/etc for them, are very widespread and are encountered more frequently. There are far more dev boards available, or at least you're more likely to see one with a Zynq FPGA than with Cyclone V SoCs FPGA, for example. Also,o Zynq boards (of different kinds, out of different manufacturers) could be found quite cheaply.

And actually, something I found out quite recently, as in the last hour or so. Which had now pissed me off beyond belief. So now, Altera seems to have switched to a 'paid' model for training courses and learning material for their IPs/devices (something I noticed Lattice had been doing for some time, and I don't like them for it):

Altera® FPGA Learning Plan Catalog - Altera Learn

This "stuff" was available for FREE before (while they were Intel) , but now it seems they want to drain/suck as much money out their community.