It's based primarily on marketing myths, is part of the reason.
Another reason is that this pairing is typically very prone to blowing speakers, specifically, the 15.
Marketing departments would have you believe that a larger diameter driver has inherently more low end than a smaller diameter one. This is simply not the case. While larger diameter drivers are usually the ones used for subwoofers, this has way more to do with cost and simplicity (one large speaker instead of several smaller ones for the same cone area) and the fact that the drawbacks of large diameter drivers (namely frequency dispersion in the upper mids and highs) don't matter for subwoofer frequencies.
Full range bass guitar drivers are not subwoofers. They're woofers. They need to produce mids and highs as well as lows. How much of every frequency a bass cab has depends on the design of the individual driver and the cab it's in way more than the size of the driver. For example, the SVT-410HLF has way more low end than many 15" cabs, while having less midrange. Because it was specifically designed to pump out massive low end. Adding a 15" driver to a 410 is not necessarily going to result in any more low end. And if the 15" is of comparable quality to the 410, it won't be loud enough to make that much difference anyway, as a 410, all else being relatively similar, has about twice the speaker area of a comparable 15 (there are super 15"s there that can and will blow crappier 410s out of the water, but a premium 15 cab with a crappy 410 is an unlikely pairing). This is especially the case because if you're primarily using the 410 for mids and highs and the 15 for lows, you have the opposite of the right amounts of speaker area for that purpose. Mids and highs take less energy to produce and are louder to the human ear than lows are. Hence why the fEarful designs of two/three way cabs use a singe 6" driver for the mids (and highs, if there's no tweeter) if they have a single 15" subwoofer driver for the lows, as that's all that's necessary for a balanced sound.
This leads us to the second problem: power. Say you have a 400 watt amp and two cabs of equal impedance. In this scenario, 200 watts would be going to each cab. In the case of a 410 and 115, with the 410, the power is split further between the 4 drivers, so each gets 50... whereas the single 15 faces the full brunt of 200 watts. If the drivers are of comparable quality, the single 15" is going to run into its limits way before any of the drivers in the 410 do, leading to potential for blown speakers. While sure, you can still destroy speakers with matched cabs, it's a bit less likely because with two 410s or two 15s or whatever, both your cabs will handle about the same amount of power and be the same volume.
Lastly, when you match cabs, it's easier to predict the sound of the two cabs together. If you already like how one cab sounds, another one just like it will simply be more of that sound.