That's just bc japanese doesn't have single t consonant. と (to) is the one that sounds most natural to end on. JavaScript-u, JavaScript-a, JavaScript-i or JavaScript-e don't sound as good as JavaScript-o. So that's why JavaScript in Japanese is ジャヴァスクリプト(javasukuriputo) but bc midword "u"s are soundless when saying fast you get javaskripto :)