Translation:I want most of the bowls, because half of them is not enough.
Avoid speaking in the negative. "Half of them is insufficient" is a better form.
Mi volas la plejparton de la bovloj, ĉar duono de ili estas malsufiĉa.
Shorter phrases reduce processing and maximize understanding. Is "de ili" implicit in Esperanto as in English?
Mi volas la plejparton de la bovloj, ĉar duono estas malsufiĉa.
We can eliminate the copula in Esperanto; English, has no verb inverse of "suffice".
Mi volas la plejparton de la bovloj, ĉar duono malsufiĉas.
There's no style guide for Esperanto (yet); Strunk and White doesn't directly translate, and some rules differ. Punctuation rules can probably translate from English; whereas Esperanto requires "ke" in many places where English should omit "they".
Maybe I should have clarified more.
[...] because half of them are not sufficient.
If the are were replaced with is, this would be a perfectly fine sentence, if a wee bit clunky-sounding. However, with the are in it, it sounds like the speaker is saying that particular bowls are not sufficient, as in, perhaps, they are tiny. Which doesn't make much sense.
Various reasons, but mainly, it is not good English. Your sentence seems to mean "I want the major part of the bowls because half of the major part is not enough". However, the Esperanto sentence tells us that half of all the bowls is not enough.