I would guess that $10 pricing just evolved from early SPACs issuing at $10 and others just followed.
It has no direct relevance but in the 90's, I participated in many traditional IPOs. Some boutique companies made it a practice of pricing their IPOs (with warrants) at the same price. Note that these were often lower tier companies of questionable quality. The low price made them attractive to the lower capitalized traders.
Coincidentally, last week I was looking deeper into SPACs. I culled about 180 names from my database that included "Acquisition" in the company name. Since most of them are trading in the $10 area, I assumed that most were SPACs. Have you come across any source that lists all existing SPACs? TIA.