My two cents is that Subaru's past is icing on the cake. Appreciated, but not necessary. Too much will sicken the stomach. The story of Re:Zero is centered around Emilia and the role that Subaru plays in her life. (So if anyone ever wondered why Emilia? There you go. XP)

There are already several isekai novels that document every extraneous reaction to being transported to another world...and they tend to be straightforward and one-note. They all have some degree of freakout or miss their old world or ruminate or yadayadayada.

It stagnates the pacing, slowing the story to a crawl by getting the MC stuck in his head. Standing still without moving forward kills the first chapter of any story when readers still aren't sure that they care enough to invest in this character or see their story to conclusion.

So that's more or less why we see a lot of work arounds for the MC's past. They're either people who didn't care about their old life, memory swiped so they remember jack all, or its brushed under the carpet to be dealt with at a latter date (maybe). It's all to keep the story moving.

Watching a character contemplate their circumstance and question reality may work the first time, but if you really wanted to kill the isekai premise dead, watch them do it every single time. :P