I wasn't around back then but I will just make random assumptions and extrapolate to look smart
Hype for game trailers wasn't as huge so a lot of it might've been word of mouth. Niche communities could become outliers with more hype due to posts on ancient forums or friend groups who wanted to play together
Natural Selection also had asymmetrical combat which it handled in a different way, but would definitely make the player base more compatible. A lot of FPS at the time were symmetrical and often homogenous in how combat went (think CS, CoD, etc where everyone had similar/mirrored access to mechanics and loadouts).
Movement tech was also important in NS so a lot of ppl probably found learning the TF2 tech enjoyable when they transitioned