The reason seems to be historical as explained by Nardog in this answer on ELU.
However, most words that end in /r/ in General American English (GAE) usually end in a schwa in Standard Southern British English (SSBE) because SSBE is non-rhotic. 'Non-rhotic' means that only prevocalic (before a vowel) R's are pronounced in SSBE. For example, the R in real is prevocalic, so it's pronounced in British English whereas the R in hear is post-vocalic (after a vowel) and there's no following vowel, so the R in this instance is silent in SSBE. General American English (GAE) on the other hand is rhotic, meaning the R's in all positions are pronounced. So the R in both real and hear are pronounced in GAE.
Most of the time SSBE replaces the R with a schwa which in GAE would be pronounced, so better is pronounced /ˈbɛtər/1 in GAE, but /ˈbɛtə/ in SSBE.
The diphthongs /ɪə, ʊə, eə/ mostly occur before an R, and the R is silent in SSBE which gives us /ɪə, ʊə, eə/ in SSBE; however, the R is pronounced in GAE which gives us /ɪər, ʊər, eər/ in GAE (Also read the Wikipedia article on GAE diphthongs).
- I used /ˈbɛtər/ for the sake of simplicity.