The decreasing wager volume in terms of BTC can be attributed to Bitcoin's price increasing. For example, take a look at bustadice's wager volume below. It may look like April 2021 was a terrible month with only 815 BTC in volume, but in terms of purchasing power (using USD as a proxy) it saw almost exactly as much volume as April 2019 two years earlier, even though that month saw 9,598 BTC wagered.
2017-10 848.31 BTC $4,690,789.68
2017-11 3,929.28 BTC $28,751,338.85
2017-12 826.33 BTC $12,316,651.35
2018-01 1,792.40 BTC $22,561,725.44
2018-02 1,939.99 BTC $18,696,744.48
2018-03 2,853.28 BTC $25,302,626.43
2018-04 2,040.41 BTC $16,648,595.05
2018-05 3,582.37 BTC $30,004,211.72
2018-06 4,753.43 BTC $31,482,905.49
2018-07 10,500.58 BTC $76,456,679.96
2018-08 36,744.34 BTC $241,887,569.44
2018-09 11,103.75 BTC $73,439,157.14
2018-10 7,837.70 BTC $50,509,250.67
2018-11 7,661.29 BTC $39,707,334.95
2018-12 15,487.18 BTC $56,495,931.55
2019-01 9,144.02 BTC $34,027,255.60
2019-02 8,350.60 BTC $30,868,933.60
2019-03 15,290.14 BTC $59,985,294.58
2019-04 9,597.95 BTC $48,921,761.59
2019-05 7,712.54 BTC $53,867,284.64
2019-06 2,019.44 BTC $18,717,393.27
2019-07 2,564.21 BTC $27,052,666.68
2019-08 2,880.79 BTC $30,543,839.57
2019-09 3,868.87 BTC $38,006,698.50
2019-10 4,318.74 BTC $35,984,699.74
2019-11 3,144.09 BTC $27,111,486.66
2019-12 5,062.35 BTC $36,751,327.06
2020-01 4,204.47 BTC $34,312,909.37
2020-02 8,333.32 BTC $81,412,569.99
2020-03 9,590.91 BTC $66,915,423.13
2020-04 10,760.62 BTC $75,512,969.96
2020-05 15,341.13 BTC $141,031,858.44
2020-06 4,153.90 BTC $39,130,837.02
2020-07 7,018.54 BTC $67,437,557.18
2020-08 5,052.84 BTC $58,560,793.44
2020-09 3,223.89 BTC $34,478,268.75
2020-10 3,135.75 BTC $37,328,851.97
2020-11 2,711.34 BTC $44,700,728.68
2020-12 3,623.63 BTC $78,761,265.51
2021-01 1,693.10 BTC $58,981,985.51
2021-02 924.67 BTC $41,345,703.62
2021-03 1,285.19 BTC $71,212,441.23
2021-04 815.31 BTC $46,692,152.05
2021-05 1,286.82 BTC $54,325,255.35
2021-06 450.08 BTC $16,481,685.44
Here are the same figures for bustabit:
2018-01 3,089.54 BTC $33,534,063.10
2018-02 107,460.39 BTC $1,013,772,841.97
2018-03 7,040.28 BTC $62,016,295.49
2018-04 6,957.38 BTC $55,564,454.23
2018-05 7,680.04 BTC $64,661,108.23
2018-06 9,541.23 BTC $64,589,401.62
2018-07 54,385.34 BTC $402,355,448.74
2018-08 17,366.30 BTC $112,679,802.01
2018-09 11,283.51 BTC $73,615,176.20
2018-10 16,285.33 BTC $104,850,712.34
2018-11 22,352.73 BTC $122,343,140.51
2018-12 38,466.22 BTC $146,480,162.93
2019-01 19,846.31 BTC $72,908,981.05
2019-02 23,801.41 BTC $87,806,398.66
2019-03 21,570.02 BTC $84,631,318.92
2019-04 18,975.73 BTC $97,910,813.00
2019-05 20,063.16 BTC $142,516,480.49
2019-06 16,467.29 BTC $153,249,874.51
2019-07 12,597.00 BTC $134,402,184.38
2019-08 10,668.02 BTC $113,280,154.82
2019-09 9,765.15 BTC $95,351,363.42
2019-10 18,544.82 BTC $154,142,807.11
2019-11 19,085.73 BTC $158,187,514.86
2019-12 19,855.11 BTC $143,791,272.54
2020-01 22,261.18 BTC $187,543,464.56
2020-02 15,844.87 BTC $153,433,733.74
2020-03 30,335.11 BTC $198,273,711.71
2020-04 34,272.85 BTC $239,859,586.74
2020-05 23,329.71 BTC $210,304,271.17
2020-06 26,727.18 BTC $252,285,895.91
2020-07 18,769.17 BTC $179,880,168.04
2020-08 20,396.90 BTC $237,299,718.21
2020-09 16,726.39 BTC $177,937,167.73
2020-10 30,828.70 BTC $374,435,114.43
2020-11 26,277.45 BTC $404,906,013.76
2020-12 10,826.68 BTC $231,965,071.50
2021-01 6,717.85 BTC $231,578,179.66
2021-02 6,015.26 BTC $270,139,312.71
2021-03 6,068.65 BTC $336,645,129.62
2021-04 4,365.19 BTC $251,112,542.55
2021-05 5,916.51 BTC $275,599,127.62
2021-06 2,423.29 BTC $87,302,287.68