By my count there have been 9 solar sail demonstration missions launched, the first being Cosmos 1 in 2005 and the most recent being the NEA Scout launched just a few months ago.

However, the vast majority of these missions have either outright failed or been a dubious success at best. The only one which has been truly successful and demonstrated practical use of a solar sail was IKAROS in 2010, which operated successfully until 2015.

It only takes one success to prove that the concept works, but the large number of failures seems to have tempered expectations and dampened enthusiasm somewhat.

There was a proposed follow-up to IKAROS called OKEANOS intended to go to the Trojan asteroids, which would have been the first real use of a solar sail for a scientific mission.

It was a finalist for ISAS's consideration in 2019, but ultimately lost to LiteBIRD. Unfortunately there just isn't enough space science funding for all the missions people would like to do, so a lot of neat stuff gets passed on.