Running your internet and your business phone system through separate providers can seem harmless, until something breaks and each provider points to the other as the cause.
Bundling both with one provider removes that finger pointing entirely. One point of contact, one bill, and a provider who's accountable for the whole experience instead of just their piece of it. It also tends to reduce cost, since providers can offer better combined pricing than two separate vendors ever will.
The exception is if your internet provider genuinely can't support quality VoIP service. In that case, the fix isn't necessarily splitting providers, it's finding one who can competently manage both.