Here I will Compare Eldritch blast to what it has been compared to the past, the Heavy Crossbow. I will list what makes it superior to the heavy crossbow.

1. the Heavy Crossbow has the loading property

This almost bankrupts the whole argument  entirely. While, Technically it does more damage with 1d10+ Dex, at higher levels it requires a feat to keep up, which is a major investment, and you only get one extra attack, while eldritch blast gets up to 4 rays. And The warlock can just pick up agonizing blast, at second level, which matches it by one level, and at level 1 1d10 damage is usually enough. The only way to match the extra rays is going full fighter, but even then it will lag slightly as it expends its first ASI on Crossbow expert.  and the extra 3 attack is only reached at 20th level. 20th level!! This doesn't even account for you don't need to keep investing in warlock to get the extra rays.

2. Force damage

While A Heavy Crossbow damage type is easily resisted or immune, at higher levels, thanks to fiends and other  high level monster.  non magical piercing damage is resisted by 33 monsters, and 42 are immune to such non magical damage. the fighter will have to pick up a magic heavy crossbow to keep pace, with not just the warlock, but the other party members.

In contrast, Force damage is resisted by none, and only 3 are immune to it, with 2 being adventure-specific. Eldritch blast will stay reliable for long times, at at higher levels you probaly won't encounter them at all.

Why eldritch blast is so good:

Invocations. 

Eldritch Blast is a Warlock only cantrip and the design was so Warlocks could invest this as to be a consistent blaster. Warlocks are primarily casters but while they have some good spells they have a very limited amount of casting and, with exception of hexblades, their non-magic combat sucks. So to balance this they got Eldritch Blast and invocations.

Most Warlocks will rely on this. Even at base it is the best blaster cantrip in the game since you can eventually spread the attacks across multiple enemies and is second-highest in damage and the least resisted and least-immune damage type. However, with invocations it gets better since you can get the following:

- Agonising Blast, each beam of your Eldritch Blast (and you get up to 4) does additional damage equal to your Charisma mod. By later levels with maxed charisma that's 4d10+20 force damage.

- Lance of Lethargy, once per turn if you hit a creature with Eldritch Blast you can impose a -10 speed penalty to it which last until the end of your next turn.

- Grasp of Hadar, once per turn if you hit a creature with Eldritch Blast you can pull the creature closer by 10 feet.

- Repelling Blast, each time you hit a creature with Eldritch Blast you can push them away 10 ft.

- Eldritch Spear, Eldritch Blast now has a range of 300 ft. Combine with the Spell Sniper for 600 ft range. If you're a Sorc multiclass you can use Distant Spell metamagic for 1,200 ft range!

With the invocations you get the ability to blast from a long range and have control over enemy movement. Push or pull them into hazards, slow them down etc. This pairs extremely well with AoE effects that trigger damage when creatures "enter" it and also when they start their turn in the area- you can pull them out, push them in to re-trigger damage and slow them down making it easier to keep them within the area.

The cantrip also pairs well with Hex (or hunters mark), since that spell triggers 1d6 on any attack, and each beam of the eldritch blast is a separate attack. That's an additional 4d6 damage if you hit with all beams.

Warlocks are mostly intended to be eldritch blasters with some occasional bit of spells. You can take other invocations and try pact blade but unless you're Hexblade, you'll never outmatch the reliable and consistent damage output of being an eldritch blaster.

Due to this design they were given the best combat cantrip in the game.