There are many reasons. Some more virtuous than others.
- Laziness -- Some people simply don't think about what they're getting themselves into when they get "into" several breeds at once or shortly one after another. When the time comes to maintain several pens, manage matings to produce the best crosses within breeds, etc. they don't want to put in the work it takes.
- Overcrowding / Inadequate Facilities -- Some people want more turkeys than they should realistically be keeping with the facilities and resources they have available. They can't, with what they have, separate the birds so they just leave them together.
- Resulting Birds Aren't Expected to Contribute to Future Gene Pools -- Some people enjoy keeping several breeds of heritage birds, but slaughter all offspring. In this case the offspring are never expected to contribute to future matings so it "doesn't matter" if they're crosses. In theory it's not so bad, but people don't always follow through with even their best intentions sothe crosses can end up breeding animals despite it all.
- Experimentation / Curiosity -- People like to see what they'll get if they cross various breeds. Some may be trying to "create" a new breed with select traits from two or three others.
- Hybrid Vigor -- Again, when the offspring are intended for slaughter some may try to get a slight edge in grow out periods, FCR, etc by crossing in hopes the resulting offspring will exhibit hybrid vigor.
- Novelty -- To me the real novelty is in a really nice purebred Tom; straight, strong legs, striking color, substantial size, calm demeanor... these are the things that make me ohh and ahh. But some people, like Steve pointed out, think odd colors that can come from crosses are novelties. Some even use -- or try to use -- the concept of these being "rare" colors as a get-rich-quick-scheme.