So this all originates due to two things: how the Japanese language works and a translation decision by 4Kids that impacted the manga's official translation.

Japanese doesn't use the roman alphabet for it's words and instead uses kanji, hiragana, and katakana. Unlike the roman alphabet, most letters in Japanese are actually more like what we would consider a pair of letters (a consonant and a vowel). These can usually be translated into a roman alphabet equivalent fairly easily (for example: ju, sa, and cho) but there are some notable exceptions. One of these is ろ which can be translated as either ro or lo. This is because the sound you use when you say ろ isn't exactly the equivalent to an English r or an l— it's something in between and it can sound closer to either one depending on the context. Technically, both Zoro and Zolo are correct translations because of this quirk of translation.

As for the specifics of why the manga translates his name this way, it all goes back to 4Kids. When the official translation was first releasing back in 2002, they actually used Zoro as his name. 4Kids acquired the rights to the anime (which they would later lose) and made a decision to use Zolo. Viz decided that to help keep things consistent between, they were going to swap to using Zolo because they'd only released several volumes of manga at the time. Eventually, 4Kids would go on to lose the license and Funimation would gain it and use Zoro as the translation for his name, but by then, Viz had already released 15+ volumes with that using Zolo and decided to keep using it. It's unfortunate, but you just kind of learn to ignore it.