That is because the AP has separate connectors for each radio (for the outdoor AP's). This gives you the flexibility to select a different antenna for each radio, which is needed if you have very directional antenna's that typically don't come in dual-band.
If you plan to mount an AP-274 with D100, you probably are better off with the AP-277 which is 80 degrees vs 90 for the D100. The built-in antenna reduces the cost for mounting.
If you really want to mount a single external antenna, you might find third-party diplexers/combiners that combine the two radios into a single dual-band antenna output. You will need one diplexer per antenna chain, so 3 for an AP-274.