If You put all Your variables equal to 1, the the equation equals locally, p1=1, infinity.
Numerically You can integrate 0..1 and 1…, but not 0…1.1.
Therefore with other real and positive variables the integral is expected to approach infinity.
$Assumptions is not cumulative. The way you give the assumptions, only the last one is kept. Also, with p1 > 0 && Element[p1, Reals], declaring p1 to be real is superfluous: p1>0 implies already that p1 is real. It would be more meaningful to assume 0<a<b.