While for an authoritative answer to this I believe you'd have to ask the manufacturers directly (unless we happen to have someone here on the site who works for one of them), there is a pretty big plausible reason why so few multiband radios include 220 MHz capability.
The band is allocated to amateur radio mostly in the United States and Canada. (Source: Wikipedia.) Outside of ITU Region 2, the only country listed as having an allocation there is Somalia. On the other hand, with minor differences in frequency ranges for which it is trivial to implement restrictions in software only, two meters and 70 cm are both allocated to amateur radio practically worldwide.
Since 220 MHz also is not harmonically related to the band pair two meters and 70 cm, adding it to an existing design would probably require a fairly significant outlay in new electronics, filters etc. You would, at the very least, need to fix up the receiver so it performs well on the band, add the ability to tune the VFO to the band (assuming you don't already have a wide coverage VFO) and add a proper band pass filter to filter the transmitted signal. Other steps might be needed as well depending on the particular transceiver design. There is also the related problem that a 2m/70cm antenna is not resonant at 1.25 meters, which increases the risk of damage due to mistakenly transmitting into an unmatched load.
It would seem that manufacturers don't consider the demand for 220 MHz capability to outweigh the cost of such design changes for a relatively limited market. The conclusion we can draw from this line of reasoning is that the reason why none of the major manufacturers include a multiband HT with 220 MHz capability in their lineup is precisely because it is a competitive market.