It all has to do with a lawyer's duty to remain independent. Here's the ABA model rule
(a) A lawyer or law firm shall not share legal fees with a nonlawyer, except that:
(1) an agreement by a lawyer with the lawyer's firm, partner, or associate may provide for the payment of money, over a reasonable period of time after the lawyer's death, to the lawyer's estate or to one or more specified persons;
(2) a lawyer who purchases the practice of a deceased, disabled, or disappeared lawyer may, pursuant to the provisions of Rule 1.17, pay to the estate or other representative of that lawyer the agreed-upon purchase price;
(3) a lawyer or law firm may include nonlawyer employees in a compensation or retirement plan, even though the plan is based in whole or in part on a profit-sharing arrangement; and
(4) a lawyer may share court-awarded legal fees with a nonprofit organization that employed, retained or recommended employment of the lawyer in the matter.
COMMENT:  The provisions of this Rule express traditional limitations on sharing fees. These limitations are to protect the lawyer's professional independence of judgment. Where someone other than the client pays the lawyer's fee or salary, or recommends employment of the lawyer, that arrangement does not modify the lawyer's obligation to the client. As stated in paragraph (c), such arrangements should not interfere with the lawyer's professional judgment.
 This Rule also expresses traditional limitations on permitting a third party to direct or regulate the lawyer's professional judgment in rendering legal services to another. See also Rule 1.8(f) (lawyer may accept compensation from a third party as long as there is no interference with the lawyer's independent professional judgment and the client gives informed consent).