Theory: Cheaper is not necessarily better

Take these cards for example. Assume your strategy is to rush to a lv6 most of us would pick Roachmon or Kabuterimon for the 1cost evo, Roachmon to enable a hard summon, Kabuterimon to have an inherit effect. Few would pick Woodmon, because you're just going to evo over it in the breeding zone, thus blocker is wasted and you have to pay 2 cost, might as well just pick Togemon which is also 2cost as well with an inherit. But Togemon has a similar effect as Kabuterimon, thus no point in paying 1 extra memory for the same effect. Similarly Vegiemon's higher DP has no value if used as an evo material.

While in a vacuum, completely ignoring what your opponent has, there certainly is no reason to not run the cheapest digimon to reach to the lv6 faster. However most non-aggro decks would opt to run a tamer that sets their memory to 3, in fact most deck base a part of their strategy on having the 3 memory to make their plays. This creates 3 scenarios: 1. Both players have no 3mem tamers, thus tries to strangle each other's memory as much as possible; 2. one player has 3 mem tamer; 3. both players have 3 mem tamers.

Scenario 1: Both no tamers

Usually both players would try to give as few memory as possible and usually passing 1-2 memory around. Asides from the 1st turn, you would at least have 1 memory thus you would need to spend 2 memory to give your opponent 1 memory. This is why many run these 2 cost lv3s to help memory management, under the same notion lv4 having 2 evo cost also helps with memory management.

Scenario 2: 1 player has a 3mem tamer

The player with the 3mem tamer would need to figure out how to spend 4 memory to put your opponent at 1. If you hard summoned Roachmon you would be at 0 memory and no lv5s cost 1 memory, thus you are forced to give your opponent at least 2 memory, in this situation, Vegiemon/Togemon 4 cost exactly puts your opponent at 1. For the player without the 3mem tamer, since you are always going to give at least 3memory, in some cases there is practically no difference between a hard summon Woodmon vs Vegiemon/Roachmon, thus you would have a blocker rather than a vanilla lv4.

Scenario 3: Both players have a 3mem tamer

Both players are essentially playing with 6 memory each turn, if only 1 card was played a 4cost hard summon literally has no difference with a 6cost hard summon, your opponent would still end with 3 memory. While certainly, you can play multiple cards, but that might require multiple hard summons which in turn reduces your hand size and miss out on the evo draws, this also forces you to commit to the board when you might not want to.

Conclusion

Cheaper is not always better, a slightly more expensive cost with some minor benefit in many cases is actually exactly the same or even better than the cheapest option, never fill your deck lv3/4/5 slots with copies of the exact same lowest cost digimon (which are usually vanillas) the minor benefits from more expensive cards might make the difference.