tl;dr they're a bit too small for SOHO and STEREO and only visible in EUV
The "campfires" are described as a few hundred km across,
The smallest of those campfires are about the size of a European country, according to Berghmans.
which means that from Earth (or SOHO or STEREO) they will subtend from one to a few seconds of arc (a few micro-radians). They are observed in extreme UV which is blocked by the atmosphere, so only space-based instruments could see them.
SOHO's imaging instrument the EIT has about a 5 arcsecond resolution.
STEREO seems to be comparable to SOHO (2000 pixels across the sun is about 500km/pixel at the centre of the disk).
Solar orbiter has a resolution of about 1 arc-second and is at half the distance, so has effectively 10 times the resolution of the other probes.
I don't know about baloon and sub-orbital instruments, but I could speculate that achieving sub-arcsecond resolution in EUV while not having an aperture so large that the incoming light fries the sensors might have been hard and expensive and required recent developments in materials and sensors coming from the semiconductor industry.