I mean, c'mon. The reasons are obvious. Well, to me at least. Here are a few.
1) The democratising nature of the internet, with all its evils, has also meant that regardless of geographical location or income level, you can hear most recorded music to ever exist. Grind is meant to continue the DIY punk spirit, and so should therefore be accessible for anyone so inclined to want to hear it regardless of how much money they have. The poor kid that found your band on YouTube or Bandcamp just might be one that's inspired to pick up an instrument, have a go and end up becoming a real freak for the style. That kid is not going to find out about your band if you release it only on vinyl with no digital option. Neither is that person on a low or fixed income with only enough money for the basics of life. You get the picture.
2) The genre is small and its listeners are dedicated enough so that a high proportion of people into grind will buy the music when they have cash available for it - this scene is not mainstream metal, where physical releases are largely just shelf trinkets for the few that buy them. Having seen how streaming/downloads leads to physical sales multiple times first hand over the last 15 years, the argument of "people won't buy it if they can just listen to it on the internet" holds zero water.
3) The cost of international postage has been rising exponentially for at least 10-12 years, and once 2020 and COVID-19 came along, it just got worse. To ship an LP from the US to Australia, for example, costs considerably more than the LP itself. Postage from local distros, while of course much cheaper, doesn't solve the issue entirely as with the waning popularity of grind here (and seemingly in many places outside of the US), it means that fewer titles are imported and less often. Any smaller country experiences this problem.
This is absolutely not to be taken as a "physical media is outdated" statement. Not at all. Records, CDs and tapes are integral to the existence of underground music, but now, a digital presence is too, for everything that comes along with that. This is not a pro-streaming argument either, and I've covered that elsewhere.