1979 was the beginning of the second period of issuing dollar gaming tokens for Nevada casinos. The reason was the Ike dollar, a new design started in the 1970's, that was supposed to provide all the circulating dollar coins the public (and the casinos) would need after silver coinage was ended and the collectors harvested all the old silver dollars. Here's how many Ike dollars were minted each year:

Circulation strikes:

1971 47,799,000
1971-D 68,587,424
1972 75,890,000
1972-D 92,548,511
1973 1,769,258
1973-D 1,769,258
1974 27,366,000
1974-D 45,517,000
(No dollars dated 1975)
1776-1976 Type I 4,019,000
1776-1976-D Type I 21,048,710
1776-1976 Type II 113,318,000
1776-1976-D Type II 82,179,564
1977 12,596,000
1977-D 32,983,006
1978 25,702,000
1978-D 33,102,890

It's certainly possible that some smaller casinos, having stocked up on Ike dollars as the end of the series approached, had sufficient that the 1979 (and 1980?) gaming tokens they had order as a hedge turned out not to be needed. Also, casino customers were not walking with many of the Ike Dollars (as they had for the earlier coins that had silver) so the casino inventories didn't deplete.

Mowery's final price list (in 2004) had Silverbird 1979 UNC dollars shown for $25 each. Vince tried to get unused tokens of every casino in the Las Vegas area, and he must have not found these in use (at least not many). $25 was a high price for UNC dollar gaming tokens in the early 2000's.

The Susan B. Agony dollar that replaced the Ike never caught on and not many slot machines seemed to have been modified to take them.

My suggested alternative to the appearance of large quantities of the 1979 and 1980 gaming tokens from Silverbird is that these never-used tokens, not needed when the place closed in 1982, were dumped in barrels and buried. I have no evidence of this theory, but it's a possibility.