There are some reasons:
- That rock-paper-scissors never actually worked well
- Civ7 wants commanders to be the centers of battle, which greatly restricts the free roaming cavalry tactics of Civ6, where those differences between units actually worked
- Simplification needed, at least at start, because Civ6 with it's light/heavy cavalry distinction brought the unit diversity to ridiculous level
I think Civ7 has a good foundation, from which with some updates, we could expect greatness
In Civ VII, after playing through it for a few months, I finally realized that the 'rock-paper-scissors' aspect in this game AND the 'simplification' compared to Civ VI in unit types are both products of the wild variety of ways you can modify unit factors.
Between Resource, Memento, Unique, Commander, Policy, IP Bonus and other variations, almost no infantry, ranged or cavalry/mobile unit fights with the same factor it supposedly has. I would also suspect (because I haven't taken the time to go back to earlier games and count up all possible modifiers) that the variations by unit are much larger than in previous games.
Just for an example, the basic 20-strength Warrior, in a Civ with several Iron deposits, bonuses from 1 - 2 IPs, a Tradition or two, under special conditions (defense, attack, versus Districts, etc) can easily have an actual combat strength of 30 - a 50% increase. Add in a few Bastion or Assault promotions from an Army Commander, and even a Tier 2 Spearman holds no terror for that Warrior.
The individual variations, not even counting additional variations from Unique units, means that the war-hungry player can customize and/or dramatically increase the power of his units in numerous ways - many of them not obvious to his opponents until they engage in combat. It also means that providing permanent changes to the attributes or strengths of units is much less important: if you want Anti-Cavalry infantry, infantry with an overall +10 combat increase does the same trick without requiring an entire specialized unit line.
The system, therefore, provides far more variation and customization overall than previous games did - it's just not laid out neatly for us in "this unit fights better against cavalry in Tundra" the way previous games made it explicit. - basically, you build your anti-cavalry, pro-tundra units using the game combat system of bonuses.
Which is not to say it couldn't be made a bit more explicit: familiar old tropes like Anti-Cavalry Infantry could be made a specific bonus from Traditions or Social Policies or some other source, to be adopted specifically when your nearest neighbors are Charlemagne, Genghis and the Mauryans.