Give The Rabbi No Rest

After eating milchig, one must wait before eating fleishig.
The Noam Elimelech(Mishpatim) writes, “Chazal are lenient in this area for Shavuos”.
Poskim rely on this l’maaseh (see Piskei Teshuvos 494).

Why not be lenient after fleishig before milchig too?

Chag Sameach