Alpha-stable isotopes Why some  isotopes with positive alpha-decay energy are stable? For example, alpha-decay energy of stable 194Pt is about 1.5 MeV. 
But there is no stable  isotopes with positive beta-decay energy.
For example,  half life of 197Pt with beta-decay energy  0.7 MeV is 19 hours
 A: There are two issues at work here. Firstly, there is relatively little phase space for this decay and little choice in the matter of which nucleons go into making the alpha as the Osmium nucleus must be left in either the ground state or a only very modestly excited state. Both effects would drive a long halflife and if the resultant nucleus is left in the ground state there will be only a low energy alpha line which will straggle out in all but the thinnest layers of source material making the detection of the decay difficult.
Earlier comments
It occurs to me that the question might be 

"Why doesn't Tallium-201 exhibit alpha decay when there is sufficent energy there?". 

I believe the answer would be that angular momentum and parity conservation can not both be satisfied at once in that decay. 
Tl-201 has $\mathrm{J}^\mathrm{P}$ of $\left.\frac{1}{2}\right.^+$, Au-197 has $\left.\frac{3}{2}\right.^+$, and the alpha particle has $0^+$. To make the angular momentum work out the alpha would need to be emitted in a relative p state, but that imposes a factor of -1 on parity and we're stuck.
