To an extent, I don't think this is just about the women's game but how the English public regard football.
We love our teams. We love our club sides, and our national team, and sometimes there's schadenfreude in seeing a rival humbled, but I'm less convinced we love football for itself. I think the appetite for going to watch football as a neutral, between 2 teams whom we have no strong feelings about just isn't as great as we think.
Euro '96 was an example of this: 19k at SJP; 21k at Anfield; 22k at Nottingham. Those stadia would have been sold out 3-4 times over had England bee playing. I know someone will poitn out it was expensive, but it was also the first top class international football tournament staged in the country for 30 years. For occasional events, our fans have no problem stumping up £50+ for tickets now, and they would have done so then if the appetite for football without the visceral tribalism was there.
So, the Lionesses will get a crowd, but I'm not surprised 2 neutral countries playing a friendly attracted so few.
I'll add that I didn't know it was on either; it could just be that!