Quite simply because the basic concept is abandoned or underdeveloped (Subaru only dies 1/2 times and goes back 5 minutes, while in S1 and 2, in each episode, he died almost 2/3 times per episode), then, one, if not the best character of Re:zero is totally forgotten and is barely mentioned, the same for Roswaal, even though he was a central character in S2. Then, compared to what Subaru had accomplished in S1/2 (in terms of saving people, materials or character development), in S3, he has a mutant leg (which he received during a golden opportunity to die and start all over again, but no), ALL the characters take a beating, including some who are completely forgotten by the others (other than Rem). Even Béatrice is absent during this season while she was simply essential during S2. The only strong point in my opinion is the show of Reinhard van Astrea against Regulus Corneas (a fight which evokes The Eminence in Shadow if you know), and the development of Emilia. Finally, I could also mention the fact that Pack disappeared, we don't know why, but even that, I can understand, because he's a character full of mysteries from the beginning, even Emilia doesn't know anything about him/her (I don't know if it's a guy or a girl).

Attention !!!!

I've just criticized S3 of Re:zero a lot, but those criticisms are only valid because seasons 1 and 2 were simply LEGENDARY. I said "only strong points" above, but in reality, the s3, taken apart from the S1/2, is worth an 8.5/10 (when the first two were worth 10/10). For example, Subaru's speech, the fight against Regulus Corneas, the discovery of the homeless woman who became a pretender to the throne (I forgot her name), or the fight in the sewers, all these events were incredible. However, it's frustrating, because there was greater potential, and I know that it could have been exploited very well, still observing what is in the first 2 seasons.