Marth is the first Lord and the face of the series, so his being in was expected.
Roy was included in Melee to promote his then-upcoming game. He was added as a Marth clone, since they had no time for anything else for him. (Melee's development was notoriously rushed--13 months of development--to meet the Gamecube launch window.)
Ike was the most recent Lord at the time of Brawl's development, so for a new FE rep, it was either him or Lyn (and Lyn infamously got Assist Trophied).
Robin, again, was the most recent protagonist at the time of Smash 4's development. He was picked over the actual protagonist Chrom because Sakurai felt Robin had more to offer, and he did. Awakening is also arguably the most popular entry in the series, so that also helps.
Lucina was originally intended to be a Marth alt, but when they decided to change the properties of her sword to make her more beginner-friendly, she got split off to her own slot.
All signs point to Corrin being mandated by Nintendo to promote Fates. Even Sakurai felt that Smash 4 was already pushing it with the FE characters in the roster thus far (all five of the above), but was convinced by his team that Corrin would be a fun Fighter.
Chrom was heavily requested by Japanese players, and was thus added as a Roy Echo.
Byleth was likely picked, again, by Nintendo to promote Three Houses. Three Houses is also one of the most popular entries in the series, and I feel that there would had been less vitriol over Byleth getting in if the FE roster wasn't as big as it is.
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"Be good to yourself, because everyone else in the world is probably out to get you." - Dr. Harleen Quinzel