Histology laboratories have historically used all sorts of techniques to adhere tissues to slides. In the past, when charged slides were considered too costly for routine sectioning, laboratories relied on albumin or other proteins to help tissues adhere to slides. Commercial reagents were (and still are) available to be used as additives to the water bath. The challenge, though, is consistency. Each technologist will fill their water bath differently and add different amounts of adhesive to the bath. While one person may be generous with the adhesive, another may be stingy, making background staining difficult to control.
Adhesives do have their own limitations (eg, background staining on H&E) and may even interfere with immunohistochemistry stains.