The reason, basically, that the IRS says "no".
Generally, you can use losses from passive activities only to offset income from passive activities. You cannot use passive activity losses to offset your other income, such as your wages or your portfolio income. Portfolio income includes gross income from interest, dividends, annuities, or royalties that is not derived in the ordinary course of a trade or business. It also includes gains or losses (not derived in the ordinary course of a trade or business) from the sale or trade of property (other than an interest in a passive activity) producing portfolio income or held for investment. This includes capital gain distributions from mutual funds (and other regulated investment companies) and real estate investment trusts.
PUB 550
