Sure, Awakening, and thus Chrom, had the relevancy advantage, but he didn't get in while Awakening was fresh. He only even got in because he could be made into an echo fighter.Chrom had room to be requested. He was known to non-Smash people because of
1) The "leak" that had gotten people to expect he would happen
2) Actually appearing in Lucina's trailer saying he would get his chance another day
3) Awakening being hot potatoes at the time due to revitalizing the franchise
Chrom's deconfirmation still had room for hope and possible appearance in Smash. If anything, that tease of him being on the trailer but then it being Lucina fed a lot of Chrom fans into asking for him further.
Lyn never got traction post-Brawl because for every smash she's gotten confirmed as assist trophy extremely early on. I doubt many people voted for Lyn in the poll as a result of her already being an assist trophy. That's why despite being a hyper popular FE character, people instead lean towards Celica or Anna or whoever else hasn't gotten shafted yet.