Hi, Because one is personal and the other is business, also you pay more for business plan. You can have up to 25TB per user.
*Unlimited individual cloud storage for qualifying plans for subscriptions of five or more users, otherwise 1 TB/user. Microsoft will initially provide 1 TB/user of OneDrive for Business storage, which admins can increase to 5 TB/user. Request additional storage by contacting Microsoft support. Storage up to 25 TB/user is provisioned in OneDrive for Business. Beyond 25 TB, storage is provisioned as 25 TB SharePoint team sites to individual users.
this package also gives you Office apps and a lot more.
There is also another way to get 5TB and more on your personal OneDrive. (i've tried this before with 2 account), but it's a bit tricky. so you can have 2 Microsoft accounts, buy 1TB (2$ per month) for each of them. make a folder in one of the accounts, share it with the other one. in the other OneDrive account add that shared folder to your library and you will be able to seamlessly use 2TB of accumulated storage with ease :)