Because people in music stores and marketers have been (erroneously) telling bass players since the beginning of time that that's what they need: i.e., 15" speakers for lows and 10" speakers for mids and highs. It sounds intuitive, but it simply isn't true for modern bass cabinets. The only time that rule holds is when you are using more than one amp to send the higher frequencies to one or more speakers that are specifically designed to reproduce only mids and highs, and sending the low frequencies to one or more speakers designed specifically to reproduce lows, as in a PA system or a bi-amped bass system designed for this purpose. But modern bass cabinets, whether loaded with 10s, 12s, or 15s, are all designed to be "full range," and there is no general tendency for 15s to sound deeper/lower or 10s to sound bright. There are plenty of bright-sounding 15s and deep-sounding 10s.