I think it's because the final two vowel sounds, 'e' and 'i', are created using the same basic mouth/tongue positions, and thus, to differentiate the two sounds

I think it's because the final two vowel sounds, 'e' and 'i', are created using the same basic mouth/tongue positions, and thus, to differentiate the two sounds, you exert more force in controlling your muscles. On the other hand, with 'e' and 'o', the mouth positions are so different, that you can be a lot more loose with the vowel sounding and still get the final vowel combination to come out right. I hope that makes sense.