I'm looking at the word 連続体(れんぞくたい) (I'm sure there are better examples, that's just the one that made me wonder).

I find it rather clunky, whereas 「れんぞくだい」 would be much easier to pronounce.

With a lot of kanji, there exist both "hard" and "soft" readings (e.g. 大(たい) and 大(だい)), but, at least by my dictionary, 体 can only be read as a "hard" たい.

To give a maybe more common word as example: 液体(えきたい) (Though I find that a bit easier to pronounce than 連続体)

Is there any reason for this? Or are these terms just not used enough or old enough to get softened? Is there some "rule" to softened readings, or are they just a result of language evolution?