Bendecir y Predecir

It's just the way that the language evolved over hundreds of years. Bendecir and Predecir actually have different patterns for their past participles:

Bendecir/Maldecir have two alternative forms. Rather than following Decir's irregular "dicho" form, they have a regular "bendecido/maldecido" and a differently irregular "bendito/maldito" form. This last irregular form is closest the the Italian/Latin forms of "benedetto/maledetto". If I can make a guess, it survived this way because of the religious influence that Latin and the Roman Catholic church has had over these two words, making them more likely to be written down and therefore not change. I'm just speculating here.

Predecir/Contradecir/Desdecir take an irregular form similar to "Decir", becoming predicho/contradicho/desdicho.

All of these forms take the gerund (present participle) in the same way, taking the usual -iendo form and the "e" in the "decir" root gives way to an "i": "___diciendo.

More info:
https://www.wordreference.com/conj/EsVerbs.aspx?v=bendecir

https://www.wordreference.com/conj/esverbs.aspx?v=predecir