There are many reasons donors might choose to donate elsewhere instead of to the Effective Altruism Funds. Here are a few examples:
They have inside information on promising opportunities
Donors may think that their donations will have a higher expected value than donations made through the Fund if they think they have better information than Fund managers in specific cases. For example, a donor might have inside information on a particular cause or organization, or an unusually strong personal network, such that their knowledge and access to expertise would provide them with an advantage.
Their preferred opportunities aren't available
Donors might not want to donate through EA Funds if opportunities they wish to support are not represented by the current Funds. For example, there could be no Fund for the cause area they are focusing on, or their values could differ enough from those of the Fund managers that the Funds' donations might not represent their values. For example, donors with strong views on the value of saving lives versus the value of improving lives may choose to allocate their donations directly to charities that work in one area or the other.
They prefer to donate through a non-EA-Funds mechanism
Behind the scenes, the structure of EA Funds operates similarly to a donor-advised fund (DAF) with the Effective Ventures Foundation (EVF) operating as the DAF. While this setup has a number of advantages, it also requires placing trust in EA Funds to donate the funds as intended. In addition, there may be unexpected edge cases that would require EA Funds' discretion to resolve. Donors that are not comfortable with this level of trust in EA Funds should consider donating through another mechanism.