I think I get your last point, too.
Marth's forte is speed and "tipping", and when making Roy, they took those away while giving him a small bit of power. However, that damaged Roy greatly, and he's near the bottom of the tier list compared to Marth.
Well if you want to get technical, there are the differences between Marth and Roy.
-Both characters have a sweetspot on their sword, Marth on the tip and Roy on the base.
-Roy has higher falling speed; meaning his recovery capabilties are worse but has a stronger SHFFL to compensate
-Roy has a marginally slower dash speed.
-Some of Roy's attacks like his neutral aerial and smash attacks are marginally slower.
-Roy's up smash hits multiple times and has a extremely situational spike at the tip, Marth's simply inflicts more damage at the tip.
-Marth's down tilt knocks the opponent horizontally, while Roy's knocks them vertically.
-Roy, on a technical standpoint, actually boasts slightly less power for a majority of his attacks, with a few exceptions like down smash and up throw.
-Marth's down aerial has spike/meteor smash properties at the tip, Roy's has a smaller hitbox roughly at the base of his torso and weaker knockback.
-Marth's Shield Breaker is weaker, but it charges fast with no recoil. Roy's Flare Blade has a slower charge and recoil when fully charge, but is stronger.
-Marth's Dancing Blade and Roy's Double-Edge Dance are similar, the main differences is that Dancing Blade increases in damage while Double-Edge Dance decreases in damage with each successful input, and on the third attack tilted down Dancing Blade inflicts a spike while Roy's hits multiple times with a fire effect.
-Marth's Dolphin Slash inflicts a single hit that is based on the position of the attack, the sooner the stronger. Roy's Blazer has multiple hit properties, the direction can be manipulated, is slower, and it's knockback is based on weight instead of damage (you can actually KO Jigglypuff at 0% with the attack). Both can be reversed.
-Marth's Counter always does 7% damage, while Roy's has a slower start up and smaller duration but inflicts 1.5x amount of damage.
The main reason why Roy fails to gain a lot of success is that his Sword of Seals properties being based around the base, meaning that he generally has a much harder time inflicting damage. Other problems include his worse recovery and bad aerial game. Marth's tip generally gives him the range and safety to KO opponents and potentially combo attacks, while because of Roy's hitbox properties he cannot rely on the same thing and instead needs to preform repetitive chain grabs and down tilts to make progress. He's generally outclassed by Marth because of those characteristics, though at the same time Roy boasts a few characteristics that are better then Marth, like being arguably faster then Marth due to his stronger SHFFL capabilities and can pressure edge guarding better due to the multiple hit properties of the Blazer.
This is of course based on the Melee incarnation, when Project M version 3.0 comes around this may or may not be relevant (leaning on the latter).
